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DESCRIPTIVE MINERALOGY 



DESCRIPTIVE 
MINERALOGY 



BY 

WILLIAM SHIRLEY BAYLEY, Ph.D. 

FBOFBSSOB OP OEOLOOT, UNIVBR8ITT OF ILLINOIS 
AUTHOR OF "ELEMENTABT CRT8TALLOOBAPHT " 




WITH TWO HUNDBED AND SIXTY-EIGHT ILLUSTRATIONS 



D. APPLETON AND COMPANY 
NEW YORK AND LONDON 

1917 



Copyright, 1917, by 
D. APPLETON AND COMPANY 



PRINTED IN THE UNITED STATES OF AMERICA 



TO 
MY HELPER 

MY WIFE 

THIS BOOK IS 
DEDICATED 






PREFACE 

The following pages are presented with the purpose of affording 
students a comprehensive view of modern mineralogy rather than a 
detailed knowledge of many minerals. The minerals selected for 
description are not necessarily those that are most common nor those 
that occur in greatest quantity. The list includes those that are of 
scientific interest or of economic importance, and, in addition, those 
that illustrate some principle employed in the classification of minerals. 
The volume is not a reference book. It is offered solely as a textbook. 
It does not pretend to furnish a complete discussion of the mineral 
kingdom, nor a means of determining the nature of any mineral that 
may be met with. The chapters devoted to the processes of deter- 
minative mineralogy are brief, and the familiar " key to the determina- 
tion of species " is omitted. In place of the latter is a simple guide 
to the descriptions of minerals to be found in the body of the text. 
For more complete determinative tables the reader is referred to one 
of the many good books that are devoted entirely to this phase of the 
subject. In the descriptions of the characteristic crystals of minerals 
both the Naumann and the Miller systems of notation are employed, 
the former because of its almost general use in the more important refer- 
ence books and the latter because of its almost universal use in modern 
crystallographic investigations. The student must be familiar with 
both notations. It is thought that this familiarity can be best acquired 
by employing the two notations side by side. 

In preparing the descriptive matter the author has made extensive 
use of Hintze's Handbuch der Mineralogie. The figures illustrating 
crystal forms are taken from many sources. A few illustrations have 

vu 



viii PREFACE 

been made especially for this volume. Figures copied to illustrate 
special features are accredited to their authors. The statistics are 
mainly from the Mineral Resources of the United States. They are 
given for the year 191 2 because this was a more nearly normal year in 
trade than any that has followed. 

The author is under obligation to the McGraw-Hill Book Company 
for permission to reproduce a number of illustrations originally published 
in his Elements of Crystallography, and also for the use of the original 
engravings in making the plates for Figures 11, $$ t 71, 90, no, 114, 115, 
118, 160, 191, 194, 224, 240, and 248. 

W. S. Bayley. 



CONTENTS 

PART I 
GENERAL CHEMICAL MINERALOGY 

CHAPTER PAGE 

I. The Composition and Classification of Minerals i 

II. The Formation of Minerals and Their Alterations 17 

PART II 
DESCRIPTIVE MINERALOGY 

III. Introduction — The Elements 36 

IV. The Sulphides, Tellurides, Selentdes, Arsenides, and 

Antimonides 68 

V. The Sulpho-salts and Sulpho-ferrites 116 

VI. The Chlorides, Bromides, Iodides, and Fluorides 134 

VII. The Oxtdes 146 

VIII. The Hydroxides 179 

IX. The Aluminates, Ferrites, Chromites and Manganttes ... 195 

X. The Nitrates and Borates 205 

XI. The Carbonates 212 

XII. The Sulphates 236 

XIII. The Chromates, Tungstates and Molybdates 253 

XIV. The Phosphates, Arsenates and Vanadates 261 

XV. The Columbates, Tantalates and Uranates 293 

XVI. The Silicates: The Anhydrous Orthosilicates 300 

XVII. The Silicates: The Anhydrous Metasilicates 359 

XVIII. The Silicates: The Anhydrous Trimetasilicates 408 

XIX. The Silicates: The Anhydrous Polysilicates 426 

XX. The Silicates: The Hydrated Silicates 441 

XXI. The Silicates: The Titanates and Titanosilicates 461 

PART III 

DETERMINATIVE MINERALOGY 

XXII. General Principles of Blowpipe Analysis 467 

XXIII. Characteristic Reactions of the More Important Elements 

and Acid Radicals 483 

ix 



CONTENTS 



APPENDICES 

CHAPTER PAGE 

I. Guide to the Descriptions of Minerals 495 

II. List of the More Important Minerals Arranged Accord- 

ing to Their Principal Constituents 513 

III. List of Minerals Arranged According to their Crys- 

tallization 521 

IV. List of Reference Books 527 

Index 529 



LIST OF ILLUSTRATIONS 

FIGURE PAGE 

i. Sodium fluosilicate crystals 14 

2. Potassium fluosilicate crystals 14 

3. Cross-section of symmetrical vein 21 

4. Cross-section of vein in green porphyry 24 

5. Diorite dike cutting granite gneiss 26 

6. Vein in Griffith mine 27 

7. Vein forming original ore-body, Butte, Mont 27 

8. Druse of Smithsonite 28 

9. Geodes containing calcite 29 

10. Alteration of olivine into serpentine 31 

11. Etch figures in cubic face of diamond 38 

12. Crystal of diamond with rounded edges and faces 38 

13. Octahedron of diamond 38 

14. Principal "cuts" of diamonds 42 

15. Premier diamond mines in South Africa 43 

16. The Cullinan diamond 43 

17. Gems cut from Cullinan diamond 44 

18. The Tiffany diamond 44 

19. Sulphur crystals 47 

20. Distorted crystal of sulphur 47 

21. Copper crystal '. 53 

22. Crystal of copper from Keweenaw Point 53 

23. Plate of silver from Coniagas Mine, Cobalt 57 

24. Octahedral skeleton crystal of gold with etched faces 58 

25. Iron meteorite , 65 

26. Widmanstatten figures on etched surface of meteorite 66 

27. Realgar crystal 70 

28. Stibnite crystal 72 

29. Galena crystal 81 

30. Galena crystals 82 

31. Chalcocite crystal 85 

32. Complex chalcocite twin 85 

33. Tetrahedral crystal of sphalerite 88 

34. Sphalerite crystal 88 

35. Sphalerite octahedron 88 

36. Greenockite crystal 91 

37. Pyrrhotite crystal 92 

XI 



xii LIST OF ILLUSTRATIONS 

FIGURE PAGE 

38. Cinnabar crystals 98 

39. Group of pyrite crystals in which the cube predominates 

40. Pyrite crystals on which 0(1 1 1) predominates 

41. Pyrite crystal 

42. Group of pyrite crystals 

43. Pyrite interpenetration twin 

44. Marcasite crystal 

45. Marcasite crystal with forms as indicated in Fig. 44 

46. Twin of marcasite 

47. Spearhead group of marcasite 

48. Arsenopyrite crystals 

49. Crystal of pyrargyrite 

50. Crystal of proustite 

51 . Bournonite crystal 

52. Bournonite fourling twinned 

53. Enargite crystal 

54. Stephanite crystal 

55. Tetrahedrite crystal 

56. Chalcopyrite crystal 

57. Chalcopyrite 

58. Chalcopyrite twin : 

59. Hopper-shaped cube of halite 

60. Group of fluorite crystals from Weardale Co 

61. Crystal of fluorite 

62. Interpenetration cubes of fluorite, twin 

63. Photographs of snow crystals 

64. Zincite crystal 

65. Hematite crystals 

66. Corundum crystal 

67. Corundum crystal 

68. Corundum crystal 

69. Quartz crystal exhibiting rhombohedral symmetry 

70. Ideal (A) and distorted (B) quartz crystals 

71. Etch figures on two quartz crystals of the same form 

7 2. Group of quartz crystals 

73. Tapering quartz crystal 

74. Quartz crystal 

75. Supplementary twins of quartz 

76. Quartz twinned 

77. Cassiterite crystal 

78. Cassiterite crystal 

79. Cassiterite twinned 

80. Rutile crystals 

81. Rutile eightling twinned 



02 
02 
02 

03 

03 
10 

10 

10 

10 

12 

18 

19 
21 

21 
23 

25 

28 

31 

31 

31 

35 

39 
40 

40 

47 
SO 

53 
56 
56 
56 
59 

59 
60 

60 

61 

61 

62 

63 
69 
69 

69 

72 

72 



LIST OF ILLUSTRATIONS xiii 

FIGURE PACE 

82. Rutile twinned 172 

83. Rutile cyclic sixling twinned 173 

84. Rutile twinned 173 

85. Anatase crystal 177 

86. Anatase crystal 177 

87. Brookite crystals 178 

88. Brucite crystal 182 

89. Limonite stalactites in Silverbow mine 184 

90. Botryoidal limonite 184 

91. Pisolitic bauxite from near Rock Run 187 

92. Diaspore crystals 190 

93. Manganite crystal 192 

94. Group of prismatic manganite crystals 192 

95. Manganite crystal twinned 193 

96. Spinel twin , 196 

97. Spinel crystal r . 196 

98. Magnetite crystal 198 

99. Chrysoberyl crystal 203 

00. Chrysoberyl twinned 203 

01. Chrysoberyl pseudohexagonal sixling 203 

02. Hausmannite 204 

03. Borax crystal 207 

04. Colemanite crystals 209 

05. Boracite crystal 211 

06. Calcite crystal. * 214 

07. Calcite crystals 214 

08. Calcite crystals 214 

09. Calcite 214 

10. Prismatic crystals of calcite 215 

11. Calcite 215 

12. Calcite: twin and polysynthetic trilling. 215 

13. Calcite 216 

14. Artificial twin of calcite 216 

15. Thin section of marble viewed by polarized light.. 216 

16. Aragonite crystal 224 

17. Aragonite twin 224 

18. Trilling of aragonite 224 

19. Witherite twinned 226 

20. Cerussite crystal 227 

21. Cerussite trilling twinned 227 

22. Cerussite trilling twinned 227 

23. Radiate groups of cerussite on galena 228 

24. Dolomite crystal 229 

25. Group of dolomite crystals 230 



XIV 



LIST OF ILLUSTRATIONS 



FIGURE PAGE 

26. Malachite crystal 232 

27. Azurite crystals 233 

28. Trona crystal 235 

29. Gaylussite crystal 235 

[30. Glauberite crystal 237 

[31. Thenardite crystal 237 

[32. Thenardite twinned 237 

[33. Barite crystals 239 

[34. Barite crystals. 240 

[35. Celestite crystals 241 

:36. Anglesite crystal * 243 

[37. Anglesite crystal 243 

[38. Anglesite crystal 243 

[39. Gypsum crystals 247 

:4c Gypsum twinned 247 

[41. Gypsum twinned 248 

[42. Epsomite crystal 250 

[43. Hanksite crystal 252 

[44. Crocoite crystals 253 

[45. Scheelite crystal 255 

[46. Scheelite crystal 255 

47. Wulfenite crystal 257 

[48. Wulfenite crystal 257 

[49. Wolframite crystal 259 

50. Monazite crystal 264 

51. Xenotime crystals 265 

52. Apatite crystal 267 

53. Apatite crystal 267 

54. Vanadinite crystal ; 262 

55. Skeleton crystal of vanadinite 272 

56. Amblygonite crystal 275 

57. Lazulite crystals 276 

58. Olivenite crystal 277 

59. Skorodite crystal 286 

:6o. Radiate wavellite on a rock surface 287 

:6i. Columbite crystals 294 

:62. Samarskite crystals 297 

:63. Olivine crystals 303 

[64. Wiilemite crystal 307 

•65. Phenacite crystal 308 

[66. Garnet crystal (natural size) 310 

[67. Garnet crystals 310 

:68. Garnet crystal 310 

69. Nepheline crystal 314 



LIST OF ILLUSTRATIONS 



xv 



FIGURE 



PAGE 



70. Zircon crystals 317 

7 1 . Zircon twinned 317 

72. Thorite crystal 319 

73. Andalusite crystals 320 

74. Topaz crystals 323 

75. Topaz crystal 323 

76. Topaz crystal 324 

77. Danburite crystal 325 

78. Zoisite crystal 327 

70. Epidote crystal 328 

:8o. Epidote crystals , 328 

[81. Chondrodite crystal. . . . : 333 

:82. Datolite crystal 334 

[83. Staurolite crystal 337 

84. Staurolite crystal twinned 337 

[85. Staurolite crystal twinned. . .• 337 

[86. Sodalite interpenetration twin of two dodecahedrons 340 

[87. Prehnite crystal 344 

:88. Axinite crystal 346 

:8q. Axinite crystal 346 

:go. Dioptase crystal 347 

91. Percussion figure 348 

92. Biotite crystal 349 

93. Biotite twinned about a plane 349 

94. Etch figures 356 

95. Muscovite crystal 356 

96. Beryl crystals 360 

97. Beryl crystals 360 

[98. Cross-section of pyroxene 363 

[99. Enstatite crystal 366 

200. Wollastonite crystal 368 

201. Augite crystal 371 

202. Augite twinned 371 

203. Interpenetration twin of augite 371 

204. Diopside crystals 372 

205. Hedenbergite crystal 373 

206. Acmite crystal 376 

207. Spodumene crystal 379 

208. Rhodonite crystals 380 

209. Ampibole crystals 384 

210. Kyanite crystals 394 

211. Bladed kyanite crystals in a micaceous quartz schist 395 

212. Calamine crystals 39^ 

213. Orthoclase crystals 4*o 



xvi LIST OF ILLUSTRATIONS 

FIGURE PAG8 

214. Orthoclase crystals > 410 

215. Carlsbad interpenet ration twins of orthoclase 410 

216. Contact twin of orthoclase according to the Carlsbad law 410 

217. Baveno twins of orthoclase 411 

218. Manebach twin of orthoclase 411 

219. Section of mirocline viewed between crossed nicols 414 

220. Adularia crystal 414 

221. Albite crystals 419 

222. Albite twinned 419 

223. Albite twinned 419 

224. Twinning st nations on cleavage piece of oligoclase 420 

225. Albite twins with the crystal axis 420 

226. Position of "rhombic sections" in albite 420 

227. Diagram of crystal of triclinic feldspar 420 

228. Potash-oligoclase crystal 422 

229. Scapolite crystals 424 

230. Clintonite twinned according to the mica law 427 

231. Clinochlore crystal 430 

232. Clinochlore twinned according to mica law 430 

233. Clinochlore with same forms as in Fig. 232 430 

234. Clinochlore trilling twinned according to mica law 430 

235. Penninite crystal 430 

236. Penninite crystal twinned 430 

237. Vesuvianite crystals 433 

238. Tourmaline crystals 436 

239. Tourmaline crystals 436 

240. Cooling crystal of tourmaline 436 

241. Cordierite crystal 439 

242. Apophyllite crystals 444 

243. Heulandite crystal 447 

244. Heulandite, var. beaumontite 447 

245. Phillipsite interpenetration twin 448 

246. Phillipsite 448 

247. Harmotome fourling twinned like phillipsite 449 

248. Sheaf -like aggregates of stilbite 450 

249. Laumontite crystal 452 

250. Divergent groups of scolecite crystals 453 

251. Scolecite crystal 453 

252. Natrolite crystals 454 

253. Thomsonite crystal 456 

254. Chabazite crystal 457 

255. Chabazite interpenetration twin 457 

256. Phacolite with same form as in Fig. 254 457 

257. Analcite crystal 459 



LIST OF ILLUSTRATIONS xvii 

FIGURE PAGE 

258. Analcite crystal 459 

259. Ilmenite crystal 463 

260. Titanite crystal 464 

261. Titanite crystal 464 

262. Titanite crystal 464 

263. Simple blowpipes 468 

264. Bellows for use with blowpipe 468 

265. Candle flame showing three mantles 470 

266. Reducing flame 471 

267. Oxidizing flame 471 

268. Props and position of charcoal 473 



DESCRIPTIVE MINERALOGY 



PART I 
GENERAL CHEMICAL MINERALOGY 



CHAPTER I 
THE COMPOSITION AND CLASSIFICATION OF MINERALS 

Definition of Mineral. — A mineral is a definite inorganic, chem- 
ical compound that occurs as a part of the earth's crust. It possesses 
characters which are functions of its composition and its structure. 
Most minerals are crystallized, but a few have been found only in an 
amorphous, colloidal condition. These are regarded as gels, or solid 
colloids. 

The most essential feature of a mineral is its chemical composition, 
since upon this are believed to be dependent all its other properties. 

Chemical Substances Occurring as Minerals. — The chemical 
substances found native as minerals may be classed as elements and 
compounds. The latter comprise chlorides, fluorides, sulphides, oxides, 
hydroxides, the salts of carbonic, sulphuric, phosphorus, arsenic, anti- 
mony and silicic acids, a large series of complicated compounds known 
as the sulpho-salts, a few derivatives of certain metallic acids — the 
aluminates and the ferrites — besides other salts of rarer occurrence, 
some simple and others exceedingly complicated, and possibly many 
solid solutions of gels or of a gel and a crystalloid. In some of these 
classes all the compounds are anhydrous. In others, some groups are 
anhydrous while the members of other groups contain one or more 
molecules of water of crystallization. 

The sulphides, chlorides and fluorides are derivatives of H2S, HC1, 
and H2F2, respectively. They may be regarded as having been pro- 
duced from these compounds by the replacement of the hydrogen by 
metals. Illustrations: CU2S, CuS, NaCl, CaF2. 



2 GENERAL CHEMICAL MINERALOGY 

The hydroxides and the oxides may be looked upon as derivatives of 
water, the hydroxides through the replacement of one atom of hydrogen 
by a metal, and the oxides through the replacement of both hydrogen 

/OH 

atoms. The mineral, brucite, according to this view is Mg<Q , 

x)H 

derived from rr/rkxrv by replacement of two hydrogen atoms in two 

H(0H) Cu\ 

molecules of water by one atom of Mg. Cuprite is yO, and tenoriU 

CuO, the former derived by replacement of each atom of hydrogen in 
one molecule of water by an atom of Cu, and the latter by replacement 
of the two hydrogens by a single Cu. 

The salts of carbonic acid (H2CO3) are the carbonates, those of sul- 
phuric acid (H2SO4) the sulphates, those of orthophosphoric acid 
(H3PO4) the phosphates, those of orthoarsenic acid (H3ASO4) the arsen- 
ates, those of orthoantimonic acid (HaSbO-O the antimonates and those 
of the silicic acids, the silicates. There are, in addition, a few arsenites 
and antimonites that are salts of arsenious (H3ASO3) and antimonous 
(HaSbOs) acids. 

The principal silicic acids whose salts occur as minerals are normal 
silicic acid (H4Si04), metasilicic acid (H2Si03), and trisilicic acid 
(I^SiaOs). The metasilicic and the trisilicic acids may be regarded 
as normal silicic acid from which water has been abstracted, in the same 
way that pyrosulphuric acid is ordinary sulphuric acid less H2O, thus: 
2H2SO4- H 2 = H2S2O7. 

(HO)4Si— H20 = H2Si03, metasilicic acid 
3(HO)4Si— 4H20=H4Si308, trisilicic acid. 

Fayalite is Fe2Si04, wollastonite, CaSi03, and orthoclase, KAlSiaOs. 

The aluminates and f errites may be regarded as salts of the hypothet- 
ical acids AIO(OH) and FeO(OH), both of which exist as minerals, 
the first under the name diaspore and the second under the name 

-AlO 



goethite. Spinel is the magnesium aluminate, Mg<^ ,(MgAl204), 

X)— AlO 

and magnoferrite the corresponding ferrate MgFe204. The very com- 

/O— FeO 

mon mineral magnetite is the iron ferrate Fe<^ , or Fe304. In 

N>- FeO 

this compound the iron is partly in the ferrous and partly in the ferric 
state. 



COMPOSITION AND CLASSIFICATION 3 

There are other minerals that differ from those of the classes above 
mentioned in containing more or less water of crystallization. These 
are usually separated from those in which there is no water of crystal- 
lization under the name of hydrous salts. 

Besides the classes of minerals considered there are others which 
appear to be double salts, in which two substances that may exist 
independently occur combined to form a third substance with prop- 
erties different from those of its components. Cryolite, 3NaF # AlF3 
or Na^AlFe, is an example. The sulphosalts furnish many other 
examples. 

Further, a large number of minerals are apparently isomorphous 
mixtures of several compounds. These are homogeneous mixtures 
of two or more substances that crystallize with the same sym- 
metry, and, consequently, that may crystallize together. Their 
physical properties are continuous functions of their chemical com- 
positions. Other minerals are apparently solid solutions in one an- 
other of simple crystallizable salts, of gels, of gels and salts, and of 
gels and adsorbed substances. Among these are some of the commoner 
silicates. 

Determination of Mineral Composition. — Since the properties 
of minerals are functions of their chemical compositions, it is important 
that their compositions be known as accurately as possible. It is 
necessary in the first place that pure material may be secured for study. 
Pure material is most easily secured by making use of the differences 
in density exhibited by different compounds. The mineral to be studied 
is pounded to a powder, sifted through a bolting cloth sieve and shaken 
up with one of the heavy solutions employed in determining specific 
gravities. When the solution is brought to the same density as that 
of the mineral under investigation all material of a higher specific gravity 
will sink. The material with a density lower than that of the solu- 
tion will rise to the surface. Material with a specific gravity identical 
with that of the solution will be suspended in it. If the mixing is done 
in a separating funnel of the proper type, the materials may be drawn 
off into beakers in the order of their densities, and thus the pure mineral 
may be separated from the impurities that were originally incorporated 
with it. After the purity of the substance is assured by examination 
under the microscope, it is ready for analysis. 

The composition of the purified material is determined by the 
ordinary methods of chemistry known as analysis and synthesis. 

In analysis the compound is broken into its constituent parts and 
these are weighed; or it is decomposed and its constituents are trans- 



4 GENERAL CHEMICAL MINERALOGY 

formed into known compounds which axe weighed. From the weights 
thus obtained the proportions of the components in the original sub- 
stance may be easily calculated if the .weight of the original substance 
be known. 

In synthesis the compound is built up from known elements or 
compounds. 

If the mineral calcite (CaCOs) is decomposed by heat into lime 
(CaO) and carbonic acid gas (CO2), or if its components are trans- 
formed into the known compounds CaS04 and K2CO3, the process is 
analysis. If the known substance CO2 is allowed to act upon the 
known substance CaO and the resulting product is a substance possess- 
ing all the properties of calcite, the process is synthesis. 

Analytical Methods. — The analytical methods made use of in 
mineralogy are: (1) the ordinary wet methods of chemical analysis, 
(2) the dry methods of blowpipe analysis, in which the mineral is 
treated before the blowpipe without the use of liquid reagents except 
to a very subordinate degree, and (3) microchemical methods, per- 
formed on the stage of a compound microscope. 

Blowpipe and microchemical analyses are made use of principally 
for the identification of minerals. By their aid the nature of the atoms 
in a compound may easily be learned, but the proportions in which 
these atoms are combined is determined only with the greatest difficulty. 
The methods are mainly qualitative. 

Wet Analysis. — For exact determinations of composition the wet 
methods of chemistry are usually employed, since these are the most 
accurate ones. They are identical with the methods described in 
manuals of quantitative analysis, and therefore require no detailed 
discussion here. They are well illustrated by Prof. Tschermak as 
follows: If 734 mg. of the mineral goethite (in which qualitative tests 
show the presence of iron oxide and water) are roasted in a glass tube, 
water is given off. This when caught and condensed in a second tube 
containing dry calcium chloride increases the weight of this second 
tube by 75 mg. The residue of the mineral left in the first tube now 
weighs about 660 mg. An examination of this residue shows it to con- 
sist exclusively of the iron oxide (Fe20s). Since only iron oxide and 
water are present in goethite the sum of these two constituents ought to 
equal the original weight of the mineral before roasting. But 660+75 
= 735, whereas the original weight was 734. The difference 1 mg. is 
due to unavoidable errors of manipulation. As it is very small it may 
be neglected in our calculations. 

The results of the analysis are generally expressed in percentages, 



COMPOSITION AND CLASSIFICATION S 

which are obtained by dividing the weights of the different constituents 
by the weight of the original substance. 



Thus: 660-5-734=89.92 per cent Fe2Q3 

75-5-734=10.22 per centlfeO 



Total 100.14 

The usual methods of analysis are, however, more indirect than this, 
the components of the substance to be analyzed being first transformed 
into known compounds and then weighed. For instance, common salt 
is known by qualitative tests to contain only Na and CI. If 345 mg. 
of the pure salt be dissolved in water and the solution be treated with 
silver nitrate under proper conditions a precipitate of silver chloride 
is formed so long as any sodium chloride remains in the solution. The 
silver chloride is separated from the solution by nitration. It contains 
all the chloride present in the 345 mg. of salt. After drying, its weight 
is determined to be 840 mg. The solution from which the silver chloride 
was separated contains all the sodium that was originally present in 
the salt, but now it is in combination with nitric acid. It contains 
also any excess of silver nitrate that was added to precipitate the chlorine. 

NaCl + AgN03 = AgCl + NaN0 3 

salt reagent precipitate filtrate 

The nitrate is now treated with hydrochloric acid to precipitate 
the excess silver. The silver chloride precipitate is removed by filtra- 
tion, leaving a solution containing sodium salts of nitric and hydro- 
chloric acids besides some free acid of each kind. Sulphuric acid is 
now added and the whole solution is evaporated to dryness. The free 
acids are driven off by the heat and the sodium salts are transformed 
into the sulphate, Na2S04. The residue consisting exclusively of Na2SC>4 
is now found to weigh 419 mg. 

The 345 mg. of salt have yielded 840 mg. of AgCl and 419 mg. of 
Na2S04. The silver chloride is known to contain 24.74 per cent of 
chlorine and the sodium sulphate 32.39 per cent of sodium. The 840 
mg. of AgCl contain 207.8 mg. of chlorine, and the 419 mg. of Na2S04 
contain 135.7 m 8- °f sodium. Hence 345 mg. of salt yield 

207.8 mg. or 60.23 per cent CI, 
and 135.7 m g- or 39-34 per cent Na 

343.5 mg. 99.57 per cent 



GENERAL CHEMICAL MINERALOGY 



Records of Analyses. — The composition of minerals like that of 
other chemical compounds is determined in percentages of their com- 
ponents and is recorded as parts per ioo by weight. A weighed quantity 
of the mineral is analyzed, the products of the analysis are weighed and the 
percentage of each constituent present is found by dividing its weight 
by the weight of the original substance, as has already been indicated. 

In chemical treatises the results of the analyses are usually recorded 
in percentages of the elements present. In mineralogical works it is 
more common to write the percentage composition in terms of the 
oxides of the elements, partly because the old analyses are recorded in 
this way and partly because certain relations between the mineral 
components can be better exhibited by comparison of the oxides than 
by comparison of the elements present in them. 

The record of the analysis of a magnesik may be given as: 



28.35 per cent, 

.34 per cent, 

14.25 per cent, 

56.98 per cent, 

Total =99.92 per cent. 



Mg 

Fe 

C 

O 



- or as 



MgO=47.25 per cent, 
FeO= .43 per cent, 
C02= 52.24 per cent, 

Total =99.92 per cent. 



Calculation of Formulas. — After the determination of the per* 
centage composition of a mineral, the next step is to represent this 
composition by a chemical formula — a symbol which indicates the 
relative number of elementary atoms in the mineral's molecule, instead 
of the number of parts of its constituents in 100 parts of its sub- 
stance. 

The construction of a formula from the analytical results is simple 
enough in principle, but in practice it is often made difficult by the 
fact that many apparently pure substances are in reality composed of 
several distinct compounds so intimately intercrystallized that it is 
impossible to separate them. In the simplest cases the formula is 
derived directly from the results of the analyses by a mere process of 
division. 

The atomic weights of the chemical elements are the relative weights 
of the smallest quantities that may enter into chemical combination with 
one another, measured in terms of the atomic weight of hydrogen which 
is taken as unity, or of oxygen taken as 16. Thus the atomic weights 
of nitrogen and oxygen are approximately 14 and 16 respectively, i.e., 
the smallest quantities of nitrogen and oxygen that can enter into com- 
bination with each other and with hydrogen are in the ratio of the 



COMPOSITION AND CLASSIFICATION 



TABLE OF ATOMIC WEIGHTS 



Element Symbol 

Aluminium Al 

Antimony Sb 

Argon A 

Arsenic As 

Barium Ba 

Bismuth Bi 

Boron B 

Bromine Br 

Cadmium Cd 

Caesium Cs 

Calcium Ca 

Carbon C 

Cerium Ce 

Chlorine CI 

Chromium Cr 

Cobalt Co 

Columbium Cb 

Copper Cu 

Dysprosium Dy 

Erbium Er 

Europium Eu 

Fluorine F 

Gadolinium Gd 

Gallium Ga 

Germanium Ge 

Glucinum Gl 

Gold Au 

Helium He 

Holmium Ho 

Hydrogen H 

Indium In 

Iodine I 

Iridium, v Ir 

Iron Fe 

Krypton Kr 

Lanthanum La 

Lead Pb 

Lithium Li 

Lutecium Lu 

Magnesium Mg 

Manganese Mn 

Mcjcury Bg 



At. Weight 

27.1 

120.2 

39.88 

7406 

137.37 
208.0 

11. o 

70.02 
112.40 

132-81 ' 

40.07 

12.005 

140.25 

35.46 
52.0 

58.97 

93 5 

63 57 
162.5 
167.7 
1520 

19.0 

157.3 
69.9 

72.5 
01 
197.2 
4.00 

1635 
1.008 

114. 8 

126.92 

193. 1 

5585 
82.92 

139.0 

207 . 20 

6.94 

I75-0 
24.32 

5493 
200.6 



Element Symbol 

Molybdenum Mo 

Neodymium Nd 

Neon Ne 

Nickel Ni 

Niton Nt 

Nitrogen N 

Osmium Os 

Oxygen O 

Palladium Pd 

Phosphorus P 

Platinum Pt 

Potassium K 

Praseodymium Pr 

Radium Ra 

Rhodium Rh 

Rubidium Rb 

Ruthenium Ru 

Samarium Sa 

Scandium Sc 

Selenium Se 

Silicon Si 

Silver Ag 

Sodium Na 

Strontium Sr 

Sulphur S 

Tantalum Ta 

Tellurium Te 

Terbium Tb 

Thallium Tl 

Thorium Th 

Thulium Tm 



Tin. 



Sn 



Titanium Ti 

Tungsten W 

Uranium U 

Vanadium V 

Xenon Xe 

Ytterbium (Neoytterbium) . . Yb 

Yttrium Y 

Zinc Zn 

Zirconium Zr 



At. Weight 

96.0 

144 3 
20.2 

58.68 
222.4 

14 01 
190.9 

16.0 
106.7 

3104 
195 2 

39 10 
140.9 
226.0 
102.9 

8545 
101.7 

1504 

441 

792 

28.3 
107.88 

230 

8763 
3206 
181. 5 

1275 
1592 
204.0 
232.4 
168.5 
118. 7 

48.1 
184.0 
238.2 

5106 
1302 

173-5 
88.7 

6537 
90.6 



8 GENERAL CHEMICAL MINERALOGY 

values 14 : 16 : i. 1 The quantities that possess these relative weights 
are known as atoms. Often the apparent ratios of the elements in 
combination are different from the ratios between their atomic weights, 
but this is always due to the fact that one or the other of the elements 
is present in more than its smallest possible quantity, i.e., in a greater 
amount than is represented by a single atom. For instance, there are 
several compounds of oxygen and nitrogen known, in which the weight 
relations between the two elements may be represented by the follow- 
ing figures: 14 : 8; 14 : 16; 14 : 24; 14 : 32, and 14 : 40. If the 
second of these compounds consists of one atom each of nitrogen and 
oxygen, and these are the smallest quantities of the elements that 
can exist in combination, the several compounds must be made up thus: 

14 : 8 14 : 16 14 : 24 14 : 32 14 : 40 

N 2 NO N2O3 N0 2 N2O5 

for N can exist only in quantities that weigh 14, 28, 42 times as much 
as the smallest quantity of hydrogen present in any compound, i.e., 
the single atom, and O in quantities of 16, 32, 48, etc., times the weight 
of the single hydrogen atom. In order that even multiples of 14 and 
16 shall exist in the ratios given above, their terms must be multi- 
plied by quantities that will yield the following results; 

28 : 16 14 : 16 28 : 48 14 : 32 28 : 80 

which are the weights respectively of the numbers of atoms represented 
in the above formulas. 

If, then, the elements combine in the ratio of their atomic weights, 
or in some multiple of this ratio, the figures obtained by analysis must 
be in one of these ratios, and consequently they furnish the data from 
which the formula of the substance analyzed may be deduced. In 
gold chloride, for example, analysis shows the presence of 64.87 per cent 
Au and 35.13 per cent CI, i.e., the gold and the chlorine are united in 

the ratio of 64.87 ; 35.13 or . The combining ratio of single 

00" o 

atoms of gold and of chlorine is, however, 196.7 : 35.5, or . Evi- 

35-5 
dently in gold chloride the ratio of gold to chlorine is only one-third 

as great as is the ratio between the atomic weights of these elements, 

or the ratio of the chlorine to the gold three times as great. Hence 

1 The atomic weight of hydrogen is more accurately 1.008, when that of oxygen 
is taken as 16. 



COMPOSITION AND CLASSIFICATION 







there must be three times as much chlorine in gold chloride as would 
be represented by a single atom of chlorine, or there must be three 
atoms of chlorine in the compound, for we cannot imagine a quantity 
of gold present which is equivalent to one-third of an atom of gold. 
Gold chloride is therefore AuCU. 

We can now prove our conclusion by calculation. One atom of 
gold and three atoms of chlorine ought to combine in the ratio of 
196.7 : 106.5 (i.e., 35.5X3). If our conclusion is correct, and the 
gold chloride analyzed is AuCfe, then the quantities of gold and of 
chlorine yielded by the analysis should be in this ratio. The figures 
obtained are in the ratio of 64.87 : 35.13. Multiplying both terms of 
this ratio by 3.031 we obtain 196.62 : 106.5, which is approximately 
the ratio expected. 

In practice, the same result as that outlined above is reached by 
dividing the results of analyses by the atomic weights of the various 
elements or groups of elements concerned. The quotients represent the 
proportional numbers of the elements or groups present. If the small- 
est quotient is assumed as unity, the ratios existing between this and 
the other quotients indicate the number of atoms or groups of 
atoms represented by the latter. 

Illustrations; 



Gold Chloride Result of Analysis Atomic Weights Quotients 

Au = 64.87 per cent -5- 196.7 = 3 2 9& 
CI - 3S-J3 ■*■ 35-5 = 9896 



Ratios 
I 

3-00+ 



Tin Chloride 

Sn = 45.26 per cent -s- 117. 4 — .384 
CI = 54- 74 -*- 355 - i-54* 



1 

4.04 



The formula of the gold chloride is AuCk, and of the tin chloride, 
SnCU. 

Magnesium carbonate on analysis may yield : C = 14. 26 ; Mg =28.37; 
Fe=.34; O— 57.03; or, if recorded in the form of oxides: 002=52.24; 
MgO= 47.25; FeO=.43- From either of these results the formula is 
easily obtained by the method described. 



0=14.26-5-11.97 = 1.188=1.009; 
Mg= 28.37-^23.94= 1.186= 1.000; 
Fe= .34-*- 55.88= .006= .006; 
O-57.03-s-15.96-3.573-3.012; - 



or, 



MgC03, if we neglect the small 
quantity of iron present. 



10 



GENERAL CHEMICAL MINERALOGY 



From the second set of figures we have: 



C02= 52.244-43.89= 1.19=1; 

MgO=47-2S-5"39-90= 1.184=1; 
FeO= .434-71.84= .006; 



or. 



MgO-C02, which is the same as 
MgC03, written in a different way. 



All formulas are derived by methods like these, but in many cases 
the processes are made more difficult by the impossibility of deciding 
positively whether those substances that are present in small quantities 
are present as impurities or whether they exist as essential parts of 
the compound. 

Formulas of Substances Containing Two or More Metallic 
Elements or Acid Groups. — In the illustration given above the com- 
pounds consist of but one kind of metallic element combined with one 
kind of acid. Often in the case of minerals there are present two or 
more metallic elements, and less commonly several acid groups. When 
two metals are present in definite atomic proportions the formula is 
written in the usual manner, as CaMg(C03)2 for the mineral dolomite, 
in which calcium and magnesium are present in the ratio of one atom 
of each to two parts of the acid group CO3. Very often, and perhaps in 
the majority of cases, when two or more metallic elements are present 
in different specimens of a mineral they are not found always in the 
same proportion — the mineral may consist of isomorphic mixtures 
of several substances. For instance, many calcium-magnesium car- 
bonates are known in which the ratio of calcium to magnesium present 
is not as 1 atom to 1 atom, but in which this ratio is as 2 atoms 
to 1 atom, 3 atoms to 2 atoms, or a ratio which would have to be 
represented by irrational figures like 2.7236 atoms to 1.5973 atoms. 
Each one of these compounds properly requires a separate formula, 
as 2CaC03+MgC03, 3CaC03+2MgC03, etc., but practically the entire 
series of compounds is represented by a single symbol, thus: (Ca • Mg)C03, 
indicating that in the series we have to do with mixtures of carbonates 
of calcium and magnesium, or with complex molecules containing in 
different instances different proportions of the two carbonates. For 
greater definiteness the symbol of the characteristic element of the 
substance which is in largest quantity in the compound is usually written 
first, as (Ca-Mg)CC>3, when calcium carbonate is in excess, or 
(Mg- Ca)CC>3 when magnesium carbonate predominates. If still greater 
definiteness is desired small figures are placed below the symbols of the 
elements concerned, as (Ca2*Mgi)C03 or (Ca3 • Mg2)CC>3, to indicate 
the respective proportions present. (Ca2-Mgi)C03 signifies that the 



COMPOSITION AND CLASSIFICATION 11 

mineral thus represented contains calcium and magnesium in the 
ratio of 2 atoms of the former to 1 of the latter. 

Compounds Containing Water. — Often salts that separate from 
aqueous solutions combine with certain definite proportions of water. 
Sometimes this water combines with the anhydrous portion of the com- 
pound to form a double salt, as MgS04+7HaO, or MgSO-i-ylfeO. 
At other times a portion of the water, in the form of the group (OH), 
called the hydroxyl group, occupies the place usually occupied by 
a metallic element, and, occasionally, that usually occupied by an 
acid group, or by oxygen, as in Mg(OH)2. 

Water of Crystallization. — Double salts composed of an anhydrous 
portion combined with water are usually well crystallized. Although 
the water appears in many cases to be but loosely combined with the 
remainder of the compound it is an essential part of its crystal particle, 
for by the loss of even a portion of it the crystal system of the compound 
is often changed. Water in this form is known as water of crystalliza- 
tion, and the compounds are designated hydrates. 

The magnesium sulphate MgS04 • 7H2O forms orthorhombic crystals. 
By evaporation of a hot solution of this substance the sulphate 
MgSO* - 6H2O separates as monoclinic crystals. 

Gypsum is CaS04 • 2H2O. Its crystallization is monoclinic. When 
heated to 200 it passes into the anhydrous orthorhombic mineral 
anhydrite, CaS04. 

Water of crystallization may frequently be driven from the com- 
pound in which it exists by continued heating at a comparatively low 
temperature. It is usually given off gradually — an increase in the tem- 
perature causing an increase in the quantity of water released until 
finally the last trace disappears. In many instances such a very high 
temperature is required to drive off the last traces of the water that it 
would appear that some of it is held in combination in a different 
manner from that in which the remainder is held. Indeed, it is not at 
all certain that double salts containing water of crystallization are 
different in any essential respect from ordinary atomic molecules in 
which hydrogen and oxygen are present in atomic form. 

Combined Water. — Water of crystallization is thought of as 
existing in the compound as water because of the ease with which it 
can be driven off. Compounds in which the hydroxyl group is present 
yield water only upon being heated to comparatively high temperatures. 
In them the elements of water are present, but not united as water. 
When freed from their combinations with the other constituents of the 
compound by heat they unite to form water. Because its elements 



12 GENERAL CHEMICAL MINERALOGY 

are thought of as closely combined with the other elements in the 
molecule, this kind of water is often distinguished from water of crystal- 
lization by the term combined water. 

Brucile (Mg(OH) 2 ) and malachite (Cu 2 (OH) 2 CC>3) are minerals 
containing the elements of water. When heated they yield water 
according to the reactions Mg(OH) 2 =MgO+H 2 and Cu 2 (OH) 2 COs 
= CuO+CuC03+H 2 0. 

Combined water is not only more difficult to separate from its com- 
bination than is water of crystallization, but when the combination 
is broken the chemical character of the original substance is radically 
changed, as may be seen from the reactions above indicated. More- 
over, combined water is given off suddenly, at a certain minimum 
temperature, and not gradually as in the case of water of crystal- 
lization. 

Blowpipe Analysis. — Although blowpipe analysis serves merely to 
identify the chemical components of minerals, it is a most important 
aid to mineralogists in their practical work. 

Nearly all minerals may be recognized with a close degree of accu- 
racy by their morphological and physical properties. To distinguish 
between several minerals that are nearly alike in these characteristics, 
however, the determination of composition is often important. In 
cases of this kind a single test made with the blowpipe will frequently 
give the desired information as to the nature of some one or more of 
the chemical elements present, and thus in a few moments the mineral 
may be identified beyond mistake. 

The apparatus necessary to perform blowpipe analysis is very 
simple and the number of pieces few. These, together with all the 
reagents in sufficient quantity to determine the composition of hundreds 
of minerals, may be packed into a box no larger than a common lunch 
box. (See pp. 467-470.) 

For more refined work than the mere testing of minerals a larger 
collection of both apparatus and reagents is necessary, but in no case 
is the quantity of material consumed in blowpipe analysis as great as 
when wet methods of analysis are used. 

Principles Underlying Blowpipe Analysis.— The principal phe- 
nomena that are the basis of blowpipe work are the simple ones known 
in chemistry as volatilization, reduction, oxidation, and solution. 

For volatilization experiments charcoal sticks and glass tubes are 
used. A blowpipe serves to direct a hot blast upon the assay. The 
volatilized products collect on the cool parts of the charcoal which 
they coat with a characteristic color, or upon the cooler portions of 



COMPOSITION AND CLASSIFICATION 13 

the glass tubes. The sublimates that collect in the tubes may be tested 
with reagents or examined under the microscope. 

Some volatile substances impart a distinct and characteristic color 
to an otherwise colorless flame. These may be tested in the direct flame 
of the blowpipe. 

Oxidation and reduction experiments are usually performed either 
on charcoal or in glass tubes. Oxidations are effected in open tubes 
and reductions in those closed at one end. The products of the oxida- 
tion or of the reduction are studied and from their characteristics the 
nature of the original substance is inferred. 

The solution of bodies to be tested is often made in the usual man- 
ner, i.e., by treating them with liquid reagents, but more frequently 
it is accomplished by fusion of a small quantity of the body with borax 
(Na 2 B 4 7 • 10H2O) or miopcosmic salt ((NH^NaHPO^-tffeO). The 
molten reagent dissolves a portion of the substance to be tested and in 
many cases forms with it a colored mass. From the color of the mass 
the nature of the coloring matter may be learned. 

Although the underlying principles of blowpipe analysis are simple 
the reactions that take place between the reagents and the assay are 
often very complex. 

More explicit details of the operations of qualitative blowpipe 
analysis are given in Part III. 

Microchemical Analysis. — The processes of microchemical analysis 
are limited in their application to the detection of a single element or, 
at most, of a very few elements in small quantities of minerals. They 
are employed mainly in deciding upon the composition of a substance 
whose nature is suspected. 

The principle at the basis of all microchemical methods is the manu- 
facture of crystallized precipitates by treatment of the mineral under 
investigation with some reagent, and the identification of these pre- 
cipitates through their optical and morphological properties. 

In practice, a small particle of the mineral the nature of which it 
is desired to know is placed on a small glass plate, which may be covered 
with a thin film of Canada balsam to prevent corrosion, and is 
moistened with a drop or two of some reagent that will decompose 
it. The solution thus formed is slowly evaporated by exposure to the 
air. The plate is then placed beneath the objective of a microscope 
and the crystals formed during the evaporation are investigated. Or, 
after a solution of the assay is obtained there is added a small quantity 
of some reagent and the resulting precipitate is studied under the 
microscope. By their shapes and optical properties the nature of the 



GENERAL CHEMICAL MINERALOGY 



Fie. i. — Sodium Fluosilicate Crystals. Magnified 73 



(After Rascnbusch.) 



—Potassium Fluosilicate Crystals. Magnified 140 diam. (After Roienbusck.) 



COMPOSITION AND CLASSIFICATION 15 

crystals produced is determined, and in this way the nature of the con- 
stituehts they have obtained from the mineral particles is discovered. 

A large number of reagents have been used in microchemical tests 
each of which is best suited to some particular condition. The most 
generally useful one is hydrofluosilicic acid (H2SiFo). If small frag- 
ments of albite and of orthoclase are placed on separate glass slips, such 
as are used for mounting microscopic objects, and each is treated with 
a drop of this reagent and then allowed to remain in contact with the 
air for a few minutes until the solutions begin to evaporate, those 
portions of the solutions remaining will be discovered to be filled with 
little crystals. The crystals in the solution surrounding the albite are 
hexagonal in habit (Fig. i), while those in the solution surrounding 
the orthoclase are cubes, octahedrons or combinations of forms belonging 
to the isometric system (Fig. 2). The former are crystals of sodium 
fluosilicate and the latter crystals of the corresponding potassium salt. 
The albite, consequently, is a sodium compound and the orthoclase a 
compound of potassium. In similar manner, by means of this or of 
other reagents the constituents of many minerals may be easily detected. 
The method, however, is made use of only in special cases, when for 
some reason or other analytical methods are not applicable. 

Synthesis. — Synthesis is the opposite of analysis. By the analytical 
processes compounds are torn apart, or broken down, whereas by syn- 
thetical operations they are put together or built up. Synthetic methods 
are employed principally in the study of the constitution of minerals 
and of their mode of formation, and in the investigation of the condi- 
tions that determine the different crystal habits of the same mineral. 
The products of synthetic reactions are often spoken of as artificial 
minerals because made through man's agency. In many instances 
these artificial minerals are identical in every sense with natural minerals. 
Consequently, they may often serve as material for study, when the 
quantity of the natural mineral obtainable is too small for the purpose. 

Classification of Minerals. — Classification is the grouping of 
objects or phenomena in such a manner as will bring together those 
that are related or that are similar in many respects and will separate 
those that are different. 

Since minerals are chemical compounds whose properties depend upon 
their compositions, their most logical classification must be based upon 
chemical relationships. But their morphological and physical properties 
are their most noticeable features, and hence these should also be taken 
into account in any classification that may be adopted. Probably 
the most satisfactory method of classifying minerals is to group them, 



16 GENERAL CHEMICAL MINERALOGY 

first, in accordance with their chemical relationships and, second, in 
accordance with their morphological and physical properties. 

The first division is into the great chemical groups, as, for instance, 
the elements, the chlorides, the sulphides, etc. The second division 
is the separation of these great groups into smaller ones comprising 
minerals possessing the same general morphological features. These 
smaller groups may contain only a single mineral or they may contain 
a large number of closely allied ones. If the basis of the subgrouping 
is manner of crystallization, it follows that the members of subgroups 
containing more than one member are usually isomorphous compounds. 
Thus the subdivisions of the great chemical groups are single minerals 
and small or large isomorphous groups of minerals, arranged in the 
order in which their metallic elements are usually discussed in treatises 
on chemistry. For example, the great group of carbonates embraces 
all minerals that are salts of carbonic acid (H2CQ3). This great group 
is divided into smaller groups along chemical lines, as for instance, the 
normal carbonates, the hydrous carbonates, the basic carbonates, etc. 
These smaller groups are finally divided into subgroups according to 
their morphological properties — the normal salts, for example, being 
divided into the two isomorphous groups known as the calcite and the 
aragonite groups, and a third group comprising but a single mineral. 

In certain specific cases some other classification than the one 
outlined above may be desirable. For instance, in books written for 
mining students it is often found that a classification based upon the 
nature of the metallic constituent is of more interest than the more 
strictly scientific one outlined above, because such a classification 
emphasizes those components of the minerals with which the mining 
student is most concerned. In books written for the student of rocks, 
on the other hand, the most important determinative features of minerals 
are their morphological characters, hence in these the classification 
may be based primarily on manner of crystallization. 

In the present volume the classification first outlined is used, but 
because such a small proportion of the known minerals are discussed 
the beauties of the classification are not as apparent as they would be 
were all described. 



CHAPTER II 

THE FORMATION OF MINERALS AND THEIR ALTERATIONS 

The Origin of Minerals. — Minerals, like other terrestrial chemical 
compounds, are the result of reactions between chemical substances 
existing upon the earth. When they are the direct result of the action 
of elements or compounds not already existing as minerals they are said 
to be primary products; when formed by the action of chemical agents 
upon minerals already existing they are often spoken of as secondary, 
though this distinction of terms is not always applied. 

Quartz (SiC>2), formed by the cooling of a molten magma, is primary; 
when formed by the action of water upon the siliceous constituents of 
rocks it is secondary. 

The Formation of Primary Minerals. — Minerals are produced in a 
great variety of ways under a great variety of conditions. Even the 
same mineral may be produced by many different methods. The more 
common methods by which primary minerals are formed are: precipita- 
tion from a gas or a mixture of gases, precipitation from solution, the 
cooling of a molten magma, and abstraction from water or air by plants 
and animals. 

Deposits from Gases. — Emanations of gases are common in vol- 
canic districts. The gases escaping from volcanic vents are mainly 
water vapor, hydrochloric acid, sulphur dioxide, sulphuretted hydro- 
gen, ammonia salts and carbon dioxide, besides small quantities of other 
gases and the vapors of various metallic compounds. By the reactions 
of these with one another or with the oxygen of the air, sulphur, salam- 
moniac (NH4CI) and other substances may be formed, and by their 
reaction upon the rocks in the neighborhood halite (NaCl), ferric chlo- 
ride (FeCla), hematite (Fe203) and many other compounds may be 
produced. 

The production of minerals through the reactions set up between 
various gases and vapors is known as pneutnatolysis. Their separation 
from the gaseous condition is known as sublimation. Minerals formed 
by sublimation are usually deposited as small, brilliant crystals on the 
surfaces of rocks or upon the walls of cavities and crevices in them. 

17 



18 GENERAL CHEMICAL MINERALOGY 

The reactions by which they are produced are often quite simple. Thus 
the reaction between sulphuretted hydrogen and sulphur dioxide yields 
sulphur (2H2S+S02=3S+2H20), as does also the reaction between the 
first named gas and the oxygen of the atmosphere (H2S+0=H20+S). 
Ferric chloride may be produced by the action of hot hydrochloric 
acid upon some iron-bearing material deep within the earth's in- 
terior. This being volatile at high temperatures escapes to the air 
as a gas. Here it may react with water vapor, with the resulting for- 
mation of hematite (2FeCl3+3H 2 0=Fe 2 03+6HCl). By the action 
of carbonic acid gas upon volatile oxides, carbonates are formed, 
(Fe203+2C02=2FeCC>3+0). In other cases, however, the reactions 
are very complicated. 

Precipitation from Solution. — Nearly all substances are soluble 
to an appreciable degree in pure water. An increase in temperature 
usually increases the quantity of the substance that can be dissolved, 
as does also an increase of pressure. Moreover, the solubility of a 
salt is increased on the addition of another salt containing no common 
ion, and, conversely, is diminished in the presence of another having a 
common ion. Thus, gypsum (CaS04*2H20) is sparingly soluble in 
water, but it becomes much more soluble upon the addition of salt 
(NaCl). On the other hand, salt (NaCl) is much less soluble in water 
containing a little magnesium chloride (MgCb) than it is in pure water. 

When a solvent contains a maximum amount of any substance that 
it may hold under a given set of conditions the solution is said to be 
saturated. From a saturated solution under ordinary conditions 
precipitation results: Upon the evaporation of the solvent; the lowering 
of its temperature or of the pressure under which it exists; or the addi- 
tion to the solution of a substance containing an ion already in the 
solution. Of course, the addition of a substance which will react with 
the solution and produce a compound insoluble in it will also cause 
precipitation. 

The following table contains the results of various experiments on 
the solubility of some common minerals: 

Solubility of Various Compounds in 100 Parts Pure Water 

(The results are given in parts by weight) 

Halite (NaCl), at 7 35.68 Calcite (CaC0 2 ), in the 

Fluorite (CaFj), at 15 J° 0037 c °ld 002 

Gypsum (CaS04*2H 2 0), at 15 .250 Strontianite (SrC0 3 ) in 

Anhydrite (CaS0 4 ), in the cold .00025 the cold °o555 

Celestite (SrS0 4 ), at 14 015 Magnetite (F 3 e0 4 ) 00035 



FORMATION OF MINERALS 19 

Percentages of Various Minerals Soluble in Water at 80° 

(When treated 30 to 32 days) 

Galena (PbS) 1 . 79 Chalcopyrite (CuFeSi) 1669 

Stibnite (SbA) 5 .01 Bournonite ((Pb -Cu)SbSa). . . 2 .075 

Pyrite (FeSj) 2 . 99 Arsenopyrite (FeAsS) 1.5 

Sphalerite (ZnS) 025 

So many substances that are usually regarded as insoluble are known 
to be dissolved under conditions of high temperature and pressure that 
no substance is believed to be entirely insoluble. 

Powdered apophyllite ((HK)2Ca(Si03)2-H20), which is a silicate 
that is generally regarded as insoluble in water, is dissolved sufficiently 
in this solvent at a temperature of i8o°-i90° and under a pressure of 
10-12 atmospheres to yield crystals of the same substance upon cooling. 

Water containing gases or traces of salts is usually a more efficient 
dissolving agent than pure water. When the gases are lost, or the 
salts are decomposed by reactions with other compounds, precipitation 
may ensue. 

Parts of Various Minerals Dissolved in 10,000 Parts of Various 

Solutions 

Gold loses 1.23 per cent of its weight when treated with 10 per cent soda 
solution at 200 . 

One part gypsum (CaSO^HjO) dissolves in 199 parts of saturated NaCl 
solution. Only .4 part dissolves in 200 parts pure water. 

Pyrite (FeSa) loses 10.6 per cent of its mass upon boiling for a long time 
with a solution of Na*S. Under the same circumstances galena loses 2.3 
per cent. 

One of the commonest of the gases found in water on the earth's 
surface is carbon dioxide. This is an active agent in decomposing sili- 
cates and in dissolving carbonates, so that water in which it is dissolved 
is usually a more powerful solvent than pure water. Its dissolving 
power increases with the pressure, as in the case of pure water, but 
diminishes with increasing temperature. The action of carbonated 
water on silicates is due to the replacement of the silicic acid by carbonic 
acid and the production of bicarbonates, which are usually more soluble 
than the corresponding carbonates. The greater solubility of carbon- 
ates, like calcite, in carbonated water is also due to the formation of 
bicarbonates. For example, the action of carbonated water upon cal- 
cite (CaCOs) is as follows: 

CaC0 3 +H 2 0+C0 2 = CaH 2 (C0 3 )2. 



20 GENERAL CHEMICAL MINERALOGY 

Carbonated water is more effective as a solvent under pressure 
because of the inability of the CO2 to escape under this condition. When 
pressure is removed the CO2 escapes, or evaporation takes place, and the 
reverse reaction occurs, as: 

CaH 2 (C0 3 ) 2 = CaC03+H 2 0+C0 2 . 

The dissolving effect of carbonated water upon various carbonates 
and other minerals and the influence of pressure and temperature upon 
the solution of a carbonate are indicated in the three tables following: 

Solubility of Certain Carbonates in 10,000 Parts of Carbonated 

Water 

(The results are given in parts by weight) 

Calcite (CaCO,), at io° 10. o Siderite (FeCO s ) at 18 7.2 

Dolomite (CaMg(CO a ) a ) at 18 . 3 . 1 Witherite (BaCO,) at io° 17.0 

Magnesite (MgCO»), at 5 . . . . 13. 1 Strontianite (SrCOs), at io°. . 12.0 

Percentages of Various Minerals Soluble in Carbonated Water 

(When treated 7 weeks) 

Adularia (KAlSi,Og) 328 Apatite (Ca»(F- CI) (P0 4 )a).. . 1.821 

Oligoclase Apatite (Ca»(F • CI) (PO<),) ... 2.018 

(NaAlSi,0 8 +CaAl(SiO) 4 ). . . .533 Olivine ((Mg.Fe) 2 Si0 4 ) 2 . in 

Hornblende (complex silicate) 1.536 Magnetite (Fe 3 4 ). . . .307 to 1 .821 

Serpentine (HiMgaSijO*) 1 . 211 

Influence of Temperature and Pressure upon the Solution of 
Magnesium Carbonate (MgCOi) in Carbonated Water 
(The results are given in parts per 10,000 by weight) 
1 atmos. at 19 . . 2 . 579 parts Temp. 13 .4° under 1 atmos. . 2 .845 parts 
32 3-730 29.3 2.195 

5.6 4.620 62.0 1 .035 

75 51 20 82.0 .490 

9.0 5659 100. .000 

Precipitation from Atmospheric Water. — Rain is an active agent 
in dissolving mineral matter. Since it absorbs small quantities of carbon 
dioxide, sulphur gases and other substances as it passes through the 
atmosphere it may act upon many compounds, dissolving some, decom- 
posing others and forming soluble compounds from those that would 
otherwise be practically insoluble. Moreover, it transports the dissolved 
materials from one portion of the crust to some other portion, where, 
under favorable conditions, they may be precipitated. The rain water 
that penetrates the earth's crust, dissolving and precipitating in its 



FORMATION OF MINERALS 21 

course through the crust, is known as vadose water. It is an important 
agent in ore-formation, since it may collect mineral matter from a great 
mass of rocks and precipitate it in some favorable place, thus making 
ore bodies. 

Deposits of Springs. — Springs are the openings at which under- 
ground water escapes to the earth's surface. Much of the water flowing 
from springs is the meteoric water which has circulated through the 
crust and is again seeking the surface. In its course through the crust it 
dissolves certain materials. Where it reaches the surface some of this 
material may be dropped in consequence of (i) evaporation of the water, 
or (2) the escape of carbon dioxide, or (3) the oxidation of some of its 
constituents through the action of the air, or (4) the cooling of the water 
in the case of warm or hot springs. 

The deposits thus formed may occur as thin coatings on the rocks 
over which the spring water passes, or as layers in the bottom of the 
spring and the stream issuing from it. Among the commonest minerals 
thus deposited are calcite (CaCOa), aragonite (CaCCb), siderite (FeCOa) 
and other carbonates, gypsum (CaSO^zHaO), pyrite (FeSs), sulphur 
(S), and limonite (Fe403(OH)e). The carbonates are deposited largely 
in consequence of the escape of CO2 from the water, gypsum in conse- 
quence of cooling, and limonite and sulphur through oxidation. If the 
water contains H2S, this reacts 
with the oxygen and a deposit .4 j 
of sulphur ensues (compare -'-^^-'. : ^,- '■■ 

When the precipitation oc- .'"^A' 
curs in cracks or fissures in the "■ V'v. V "-_. 

rocks the precipitated matter l.-T_- 
may partially or completely fill _"-_" V 
the fissure, producing a vein; or, ". ^' 

the precipitated matter may fill "J " - " 
an irregular cavern forming a '.. - x-„ ■ 

bonanza. It sometimes covers " 

the walls of cavities or the sur- Fie. 3.— Cross-section of Symmetrical Veto, 
faces of minerals already exist- (After Le Nine Foster.) 

ing, giving rise to a druse. In (a) De™™c™«i ™*. W Galena, 

other cases precipitation may 

occur while the solution is dripping from an overhanging surface, 
making a stalactite, or the precipitate may fill the tiny crevices between 
grains of sand cementing the loose mass into a compact rock, 

Minerals produced by precipitation are often beautifully crystallized. 



! 



22 GENERAL CHEMICAL MINERALOGY 

At other times they form groups of needles yielding globular and other 
imitative shapes, while in still other instances they occur as pulverulent 
or amorphous masses. The fillings of veins are often arranged sym- 
metrically, similar materials occurring on opposite sides of their central 
planes in bands, as shown in the figure (Fig. 3). Some important ores 
have been concentrated and deposited in this way. 

Deposits from Hot Springs. — The water of hot springs deposits a 
greater variety of minerals than that of cold springs. Practically all 
minerals that are soluble in hot water or in hot solutions of salts are 
among them. Among those of economic value may be mentioned 
cinnabar (HgS) and stibnite (Sb2S3). 

Deposits from the Ocean and Lakes. — The water of the ocean and 
of many lakes is rich in dissolved salts. That of lakes, however, is often 
saturated or nearly so, while that of the ocean is not near the saturation 
point. Consequently, while many lakes may deposit mineral sub- 
stances, the ocean does not do so except under peculiar conditions. When 
a portion of the ocean is separated from the main body of water, it may 
evaporate and leave all of its mineral matter behind. Lakes may also 
completely evaporate with a similar result. In each case the deposits 
form layers or beds at the bottom of the basin in which the water was 
collected. 

In other instances the water brought to the ocean or a lake may 
contain substances which will react with some of the materials already 
present and produce an insoluble compound which will be precipi- 
tated. 

Of course, the nature of the beds thus formed will depend upon the 
character and proportions of the substances that were in the water. 
The ocean will yield practically the same kinds of compounds all over 
the world and the beds deposited by the evaporation of ocean water 
will be formed in nearly the same succession everywhere. In the case 
of enclosed bodies of water — like lakes or seas — in which the composi- 
tion of the water may differ, the deposits formed may also differ. 

Many of the deposits formed in bodies of water are of great eco- 
nomic importance and, consequently, are extensively worked. Prob- 
ably the most important are the beds of salt (NaCl) and of gypsum 
(CaSO^EfeO), although borax (Na2B407 • 10H2O) was formerly 
obtained in large quantity from the deposits of some of the lakes in 
the desert portions of the United States. 

In the following table are given the results of analyses of water of 
the ocean and of Great Salt Lake, in Utah, calculated on the assump- 
tion that the elements are combined in the manner indicated in the 



FORMATION OF MINERALS 



23 



column on the left. The results of the analyses of the waters of a few 
noted lakes are given in the succeeding table. 

Composition of Salts Contained in Water of the Ocean and Great 

Salt Lake 

(Parts in iooo of Water) 

I II HI 

NaCl 27.3726 8.1163 118.628 

KC1 5921 .1339 

MgClj 33625 .6115 14.908 

CaS0 4 1 .3229 .9004 .858 

MgS0 4 2.2437 3o85S 

Na 2 S0 4 9-321 

K 2 S0 4 5363 

RbCla .0190 .0034 

MgBr 3 0547 .0081 tr 

Ca«(P0 4 )i 0156 .0021 

CaCOi 0434 .0780 

FeCOi 0019 .0011 

SiOt 0149 .0024 



35-°433 



12.9427 



149.078 



I. Water of N. Atlantic off Norwegian Coast. Analyst, C. Schmidt. 
II. Average of Five Analyses, Caspian Sea at depths of from 1 m. to 640 m. 

Analyst, C. Schmidt. 
III. Great Salt Lake, Utah. Analyst, O. D. Allen. 



Percentage Composition of the Residues of a Few Lake Waters 



Dead Sea 

Lake Beisk, Siberia 

Goodenough Lake, B. C. 
Borax Lake, Cal 



CI 


Br 
1.45 


SO4 


CO3 


Na 


K 

3.24 


Ca 


Mg 


1 

SiO,. 

etc. 


64.49 


.45 




15.75 


4.09 


10.53 


tr 


22.70 


tr 


42.32 


.61 


31.32 1.01 


.07 


1.86 


.02 


7.64 




7.08 


41.41 


36.17 


6.65 


.02 


.04 


.99 


32.27 


.04 


.13 


22.47 


38.10 


1.52 


.03 


.35 


.02 



Total Solida 

(per 1000 

of Water). 



220.3 
104.7 
103.47 
76.56 



Deposits from Magmatic Water. — Equally important in depositing 
mineral matter is the water that escapes from cooling lavas and other 
molten magmas — designated as juvenile water. All molten magmas 
existing under pressure, i.e., at some distance beneath the crust, contain 
the components of water, which escape as the magma cools or when the 
pressure diminishes, whether the diminution of the pressure is due to 



24 GENERAL CHEMICAL MINERALOGY 

the escape of the lava to the surface or to the cracking of the crust. 
In its passage to the surface the hot water carrying dissolved salts pene- 
trates all the cracks and cavities in the rocks through which it passes 
in its ascent and deposits its burden of material, forming veins and other 
types of deposits. Or, its components may decompose the materials 
with which it comes in contact, replacing them wholly or in part by the 
substances which it is carrying or by the products of decomposition. 



FlO. 4. — Croas-section of Vein in Green Porphyry. The vein filling is chalcedony. 
The white splotches are feldspar crystals. The [airly uniform character of the 
rock where not affected by the vein is seen on the right side of the picture. The 
rude banding parallel to the vein is due to changes that have proceeded out- 
ward from tbe vein-mass into the rock. 

Since in many cases magmatic water contains corrosive gases, such as 
fluorine, its action on the rocks which it traverses is profound. A tiny 
crack in the rocks may be gradually widened and the material on both 
sides of it be replaced by new material, thus producing a vein which 
is sometimes difficult to distinguish from a vein made in other ways 
(Fig. 4). This process is known as metasomatism, which is one kind of 
metamorpkism. It is an important means of producing pseudomorphs 
and bodies of mineral matter sufficiently rich in metallic contents to 
constitute ore-bodies. 



FORMATION OF MINERALS 25 

Solidification from Molten Magmas. — A molten magma, such as a 
liquid lava, is probably a solution of various substances — mainly sili- 
cates — in one another, or in a hot solvent. Upon cooling or upon change 
of conditions, such as may arise from loss of gas or water or from reduc- 
tion of pressure, this hot solution gradually deposits some of its con- 
stituents as definite chemical compounds. Upon further cooling other 
compounds solidify and so on, until finally, if the rate of cooling has been 
slow, the entire mass may separate as an aggregate of minerals — such 
as constitute many of the rocks, as granite for instance, and many of the 
lavas. If the cooling has been rapid, some of the material may separate 
as definite minerals while the remainder solidifies as a homogeneous 
glass, as in the case of most lavas. Sometimes the minerals thus formed 
are bounded by crystal planes, but usually their growth has been so 
interfered with that it is only by their optical properties that they can 
be recognized as crystalline substances. The nature of the minerals 
that separate depends upon a great variety of conditions, the most 
important of which is the chemical composition of the magma. 

In some cases the minerals separating from a magma tend to segre- 
gate in some limited portion of its mass and thus produce an accumula- 
tion that may be of economic value, i.e., the magma differentiates. 
Magnetite (FeaO^, ilmenite ((Fe-Ti)203), pyrite (FeS2) and a few other 
minerals are sometimes segregated in this way in very large masses. 

Metamorphic Minerals. — Many minerals are characteristic of rocks 
that are in contact with others that were once molten. They were 
formed by the gases and hot waters given off from the magmas before they 
cooled. The hot solutions with their charges of gas and salts penetrated 
the pores of the surrounding rock and deposited in them some of their 
material. They reacted with some of the rock's components, producing 
new compounds, and extracted others, leaving pores into which new 
supplies of gas and water might enter. In some cases the entire body 
of the surrounding rock has been replaced by new material for some 
distance from the contact. Beyond this belt of most profound meta- 
morphism are other belts in which the rock is less altered, until finally in 
the outer belt is the unchanged original rock. Into the outer contact 
belt perhaps only gas penetrated and the changes here may be entirely 
pneumatolytic. Near the contact the changes may be metasomatic. 
Minerals formed by these processes near the contact of igneous masses 
are frequently referred to collectively as contact minerals. 

In other cases new minerals may be produced in rocks in consequence 
of crushing attended by heat. Hot water under high pressure 
greatly facilitates chemical changes. A part of the materials of the 



* .# 



26 GENERAL CHEMICAL MINERALOGY 

crushed rock dissolves, reactions are set up and new compounds may 
be formed. The new minerals produced are more stable than the 
original ones and have in general a greater density and consequently 
a smaller volume. The type of metamorphism that produces these 
effects is known as dynamic metamorpkism. 

Organic Secretions. — The transfer of mineral substances from a 
state of solution to the solid condition is often produced through the aid 
of organisms. Mollusca, like the oyster, clam, etc., crustaceans, like 
the lobster or crab, the microscopic animals and plants known as pro- 



Fig. 5. — Diorite Dike Cutting Granite Gneiss, Pelican Tunnel, Georgetown, Colo. 

(After Spun and Carry.) 

tozoans and algae and many other animals and vegetables abstract 
mineral matter from the water in which they live and build up for them- 
selves hard parts. These hard parts, usually in the form of external 
shells, are composed of calcium carbonate (CaCOa), either as calcite or 
aragonite, of silica (SiOz) or of calcium phosphate Ca3(PO.j)2. Although 
not commonly regarded as minerals these substances are identical 
with corresponding substances produced by inorganic agencies. 1 

Paragenesis. — -It is evident that minerals produced in the same 

1 Plants and animals upon decaying yield organic acids which may attack minerals 

already existing and thus give rise to solutions which may deposit pyrite (FeSj), 

limonite (a hydrated iron oxide) or some other metallic compound. This process, 

however, is property simply a phase of deposition from solutions. 



FORMATION OF MINERALS 27 

way will generally be found together. A certain association of minerals 
will thus characterize deposits from magmas, another association 



Fig. 6.— Vein in Griffith Mine, Georgetown, Colo., Showing Two Periods of Vein 

Deposition. (After Sfiurr and Carry.) 

(T\ - wall rock. b =- sphalerite. c - chalcopynto. 

q - comb quarts. P — pyrite. g — galena. 

Balance of vein-filling is a mixture of manganese-iron carbonates. 



Fig. 7. Vein Forming Original Ore-Body, Butte, Mont. (After W. H. Weed.) 

(0 P«ult breccia; (2) ore; (3) altered granite; (4) firsl-dasa ore; (5) crushed quarti and 
wrntte; (6) fault clay; (7) solid pyrite and bornite; (S) crushed quartz and pyrite; (9) solid 



those precipitated from water, another those produced by contact 
action, etc. This association of minerals of a similar origin is known 



28 GENERAL CHEMICAL MINERALOGY 

as their paragenesis. From a study of their relations to one another the 
order of their deposition may usually be determined. 

Occurrence. — The manner of occurrence of mineral substance is 
extremely varied, as may be judged from the consideration of the vari- 
ous ways in which they are formed. Deposits laid down in water occur 
in beds or in the cement uniting grains of sand, etc., such as the beds 
of salt (NaCl) or gypsum (CaS04 ■ 2H2Q) found in many regions. Those 
produced by the cooling of magmas may form great masses of rock 
such as granite, which when it occurs as the filling of cracks in other 
rocks is said to have the form of a dike (Fig. 5)- Deposits made by 
water, whether meteoric or mag- 
matic may give rise to veins, which 
may be straight-walled or branch- 
ing, like the veins of quartz (SKfe) 
that are so frequently seen cutting 
various siliceous rocks. When the 
veins are filled by meteoric water 
they often have a comb-structure — 
the filling consisting of several sub- 
stances arranged in definite layers 
following the vein walls (see p. 21). 
If the composition of the depositing 
solution, whether meteoric or mag- 
matic, has remained constant for a 
long time the vein may be filled 
with a single substance. If its com- 
position changed during the time 
the filling was in progress the layers 
are of different kinds. Further, it 
Fig. 8.— Druse of Smithsonite (ZnCft) deposUioncontmueduninterruptedly 
on Massive Smithsonite. the layers may match on opposite 

sides of the vein and the succession 
may be the same from walls to center. If, however, after the partial 
or complete filling of the crack it was reopened and the new crack was 
filled, the new vein when filled would be unsymmetrical if the new crack 
occurred to one side of the center of the original vein (Fig. 6). Repeated 
reopening may give rise to a vein that is so lacking in symmetry that 
it is difficult to trace the succession of events by which it was produced 
(Fig- 7)- Veins filled by inagmatic water are frequently more homo- 
geneous. 

Druses (Fig. 8) arise when deposits simply coat the walls of fissures. 



FORMATION OF MINERALS 29 

In many cases they may be regarded as veins, the development of which 
has been arrested and never completed. When the deposits coat the 
walls of hollows within rocks they are known as geodes (Fig. o). Geodes 
are common in limestones and other easily soluble rocks in which 
cavities may be dissolved. 

Gases and water under great pressure may penetrate the micro- 
scopic pores existing in all rocks and there deposit material which may 
fill the pores and cement the rocks. If the deposited material is metallic 
the rocks may be transformed into masses sufficiently rich in metallic 
matter to become ore-bodies. A body of this kind is known as an 
impregnation. It is well represented by some of the low grade gold 
ores, such as those in the Black Hills. 

When rocks are decomposed by the weather they are broken up. 



Fig. q.— Geodes Containing Calcite (CaCOi) Crystals. 

The rains wash the disintegrated substance into streams. In its course 
downward to lakes or the ocean, the heavier fragments, such as metallic 
particles, may settle while the lighter portions are carried along. 
Thus the heavy parts may accumulate in the stream bottoms. These 
materials, consisting of gold, magnetite, garnet, pyrite and other min- 
erals of high specific gravity, form a loose deposit in the stream bed 
which is known as a placer. Gold is often found in placer deposits. 
The lighter portions may be carried to the lake or sea into which the 
streams enter and may accumulate as sand on beaches and on the 
bottom near the shores as gravel, sand, silt, etc. Most sand consists 
principally of quartz, but many sands contain also grains of feldspar 
and other silicates, and sometimes other compounds. 



30 GENERAL CHEMICAL MINERALOGY 

Alteration of Minerals. — Minerals, like living things, are constantly 
subject to change. Circulating waters may dissolve them in part, 
or completely, and transport their material to a distant place, there 
depositing it either in the form it originally possessed or in some new 
form. On the other hand, the mineral substance may be decomposed 
into several compounds some of which may be carried off, while others 
are left behind. Again, the material remaining behind may com- 
bine with other matter held in the water causing the decomposition, 
and may form with it a new mineral or a number of different minerals 
occupying the place of the original one. This is in part metasomatism. 

The atmosphere may also act as a decomposer of minerals. Through 
the agency of its oxygen it may cause their oxidation, or it may cause 
them to break up into several oxidized compounds. Through the agency 
of its moisture, it may dissolve some of these secondary substances or 
it may form with them hydrated compounds. The substances thus 
formed may be dissolved in water and carried off, or they may remain 
to mark the place of the mineral from which they were derived. 

Water, containing traces of salts, or gases in solution are exceedingly 
active agents in effecting changes in minerals. Many examples of the 
alteration of practically insoluble minerals under the influence of dilute 
solutions are known. Calcite (CaCOa), for instance, when acted upon 
by a solution of magnesium chloride (MgCb) takes up magnesium and 
loses some ©f its calcium. Monticellite (CaMgSi04) when acted upon 
by solutions of alkaline carbonates breaks up into a magnesium silicate 
and calcium carbonate. Dilute solutions of various salts are constantly 
circulating through the earth's crust and are there effecting trans- 
formations in the minerals with which they come in contact. On, or 
near, the surface the transformations are taking place more rapidly 
than elsewhere because here the solutions are aided in their decompos- 
ing action by the gases of the atmosphere. 

The effect of the air in causing alteration is seen in the green coat- 
ing of malachite ((CuOH^COs) that covers surfaces of copper or of 
copper compounds exposed to its action. In this particular case the 
coating is due to the action of the carbon dioxide and the moisture of 
the atmosphere. Other substances in contact with the air are coated 
with their own oxides, sulphides, etc. 

Pseudomorphs. — When the alteration of a mineral has proceeded 
in such a manner that the new products formed have replaced it particle 
by particle a pseudomorph results. Sometimes the newly formed sub- 
stance crystallizes as a single homogeneous grain filling the entire 
space occupied by the original substance. Usually, however, the alter- 



FORMATION OF MINERALS 31 

ation begins along the surfaces of cracks or fissures in the body under- 
going alteration, or upon its exterior, thus producing the new material 
at several places contemporaneously (Fig. 10). When' the replace- 
ment takes place in this manner the resulting mass is a network of 
fibers of the new substance or an aggregate of grains with the outward 
form of the replaced mineral. 

With respect to their method of formation chemical pseudomorphs 
may be classified as alteration 
pseudomorphs and replacement 
pseudomorphs. 

Alteration Pseudomorphs. — 
Pseudomorphs of this class may 
be denned as those which retain 
some or all of the constituents of 
the original minerals from which 
they were derived. 

Paramorphs. — Pseudomorphs 
composed of the material of the 

pseudomorphed substance with- Flc . IO _ Alteration of Olivine into Ser- 
out addition or subtraction of pentine. The alteration is proceeding 
any component are known as from the surface of the crystal and 
paramorphs. from surfaces of cracts that traverse 

n ,. . .,, , it. The black specks and streaks 

Faramorphism is possible only , __.-., . . - .. 

r r J represent magnetite formed during the 

in the case of dimorphous bodies. ptoeess . (After Tschtrmak.) 
It results from the rearrangement 

into new bodies of the particles of which the original body was com- 
posed. 

Illustrations: CalcUe (hexagonal CaCOa) after aragonite (ortho- 
rhombic CaCOa); orthorhombic sulphur after the monoclinic variety. 

Partial Pseudomorphs. — The great majority of pseudomorphs 
retain a portion, but not all, of the material of the original mineral. 
They may be formed by the addition of material to the original body; 
by the loss of material from it; or by the replacement of a portion of 
its material by new material. 

Pseudomorphs formed by the addition of substance to that already 
existing are rare. The substances most frequently added in the pro- 
duction of such pseudomorphs are oxygen, sulphur, the hydroxyl 
group (OH) and the carbonic acid group (CO3 and CO2). 

Illustrations: Malachite ((CuOH) 2 C03) after copper, and argentile 
(AgzS) after silver. 

Pseudomorphs resulting from the loss of material are not common. 



32 GENERAL CHEMICAL MINERALOGY 

They are caused by the abstraction of one or more of the constituents 
of a compound. 

Illustration: Native copper after cuprite (CU2O). 

The greater number of partial pseudomorphs are formed by the sub- 
stitution of some of the components of the original mineral by a new 
material. 

Illustrations: Limonite (Fe40 3 (OH)e) pseudomorphs after siderite 
(FeC0 3 ) may be formed by the following reaction: 

4 FeC0 3 + 2O+3H2CM 4C02+Fe 4 3 (OH) 6 . 
Cerussite (PbC0 3 ) may be formed from galena (PbS), thus: 
PbS+ 4 0+Na 2 C03 = PbC0 3 +Na 2 S0 4 . 

Replacement Pseudomorphs. — Often the entire substance of a 
mineral is replaced by new material, so that no trace of its original 
matter remains. In this case the nature of the pseudomorphed min- 
eral can be discovered only from the form of the pseudomorph. 

Illustrations: Quartz (S1O2) after calcite (CaC0 3 ) and gypsum 
(CaS0 4 -2H 2 0) after halite (NaCl). 

Mechanical Pseudomorphs. — The processes described above as 
originating pseudomorphs are chemical, and the resulting pseudomorphs 
are sometimes designated chemical pseudomorphs. There is another 
class of pseudomorphs, however, in which the substance of a crystal 
has not been replaced gradually by the pseudomorphing substance. 
In this class the pseudomorphing substance simply fills a mold left by 
the solution of some preexisting crystal. Thus, if a sulphur crystal 
should become encrusted with a coating of barite (BaS04) and the 
temperature should rise until the sulphur melts and escapes, there 
would be left a mold of itself constructed of barite. If, now, a solution 
of calcium carbonate should penetrate the cavity and fill it with a deposit 
of calcite (CaC0 3 ), the mass of calcite would have the shape of a crystal 
of sulphur. Pseudomorphs of this kind are known as mechanical 
pseudomorphs. 

Weathering. — The term weathering is applied to the sum of all the 
changes produced in minerals by the action of the atmosphere upon 
them. Although nearly all minerals show some traces of weathering, 
these traces may often be detected only by the slight differences in color 
exhibited by surfaces that have been exposed for a long time to the 
action of the air when compared with fresh surfaces produced by frac- 
ture or cleavage. 



FORMATION OF MINERALS 33 

The weathering of minerals is often of great economic importance. 
Veins of sulphides and a few other compounds may be oxidized where 
they outcrop on the surface. Some of the decomposition products thus 
formed may be soluble and others insoluble. The insoluble products 
may remain near the surface while the soluble ones are carried down- 
ward by ground water along the course of the vein. Here a reaction 
may ensue between the soluble salts and the undecomposed portion of 
the vein with the result that metallic compounds may be precipitated, 
thus enriching the original vein matter and causing it to be changed 
from a comparatively lean ore to one of great richness. 

Pyrite veins on the surface are often marked by accumulations of 
limonite derived by the oxidation of the sulphide. With this may be 
mixed insoluble carbonates, silicates and other salts of valuable metals 
present in the original sulphide. Weathering may extend downward 
along the veins for a short distance, replacing their upper portions with 
the oxidized decomposition products. This portion of a vein is often 
spoken of as the oxidized zone, and this is sometimes the richest portion 
of the vein. It may be rich because less valuable substances have 
formed soluble salts and have been drained away. 

Below the oxidized zone may be another zone less rich in valuable 
compounds than the oxidized zone, but much richer than the material 
below it. The soluble decomposition products of the upper portion of 
the vein may percolate downward, and react with the unchanged vein 
matter, precipitating valuable metallic salts. Although the original 
vein matter may contain an inconsiderable quantity of the valuable 
material, the precipitation in it of additional stores of material of the 
same kind may raise the percentage of this constituent to a point where 
it is profitable to mine it. This belt of enriched ore is known as the 
zone of secondary enrichment. 

The oxidized zone extends downward from the surface to a depth at 
which the atmosphere and meteoric water become exhausted of their 
oxygen — a depth which varies with local conditions. The zone of 
secondary enrichment extends from the bottom of the oxidized zone 
to a short distance below the level of the ground water, beyond which 
solutions will diffuse and thus be carried away from the vein. Below 
the zone of enrichment the original vein-filling may reach downward 
indefinite distances. 

Since many veins exhibit the features described, it follows that the 
ore of many mines must grow poorer with depth, and that in many 
instances the richest ore is near the surface. 

Some of the changes involved in weathering and secondary enrich- 



34 GENERAL CHEMICAL MINERALOGY 

ment of sulphide veins in limestone are indicated by the following reac- 
tions in the case of a vein containing pyrite (FeS2), sphalerite (ZnS), 
and galena (PbS). 

(i) The first change produced at the surface may be the oxidation 
of the sulphides to sulphates. 

(a) ZnS+40=ZnS0 4 ; 

(b) PbS+40=PbS0 4 (anglesite); 

(c) FeS 2 +70+H20=H 2 S04+FeS0 4 . 

(2) These may react with the limestone as follows: 

(smithsonite) (gypsum) 

(a) ZnS0 4 +CaC0 3 +2H 2 0=ZnCOs + CaS0 4 -2H 2 0; 

(cerussite) (gypsum) 

(b) PbS0 4 +CaC03+2H 2 0=PbC0 3 + CaS0 4 -2H 2 0. 

(3) Some of the sulphates and carbonates carried down into the un- 
altered sulphides may react with these, yielding: 

(galena) 

(a) PbS0 4 +FeS 2 +0 2 =PbS+FeS0 4 +S0 2 ; 

(galena) (siderite) 

(b) PbC0 3 +FeS 2 +0 2 =PbS + FeC0 3 + S0 2 ; 

(galena) 

(c) PbS0 4 +ZnS = PbS+ZnS0 4 ; 

(galena) (smithsonite) 

(d) PbC0 3 +ZnS = PbS + ZnCOs. 

The PbS replacing the ZnS and deposited in the cracks in the original 
mixture of PbS, ZnS and FeS 2 increases the percentage of this compound 
in the vein and thus enriches it. 

There is also an increase in the percentage of ZnS brought about by 
the reactions between the zinc salts (1a and 2a), and the pyrite, analogous 
to those between the lead salts and pyrite ($a and 36). Thus: 

(sphalerite) 
ZnS0 4 +FeS 2 +0 2 = ZnS + FeS0 4 +S0 2 , 

(sphalerite) 
ZnC0 3 +FeS 2 +0 2 = ZnS + FeC0 3 +S0 2 . 



FORMATION OF MINERALS 35 

The zinc salts produced in reactions $c and 3<f if carried downward will 
also have the opportunity to react with the pyrite in the same way. 

If the ZnS is deposited in fissures in the vein matter this will tend to 
enrich it with zinc. 

The oxidized zone contains (smithsonite) ZnCCfo, (anglesite) PbSO^, 
(cerussite) PbCOa and (limonite) Fe2(OH)2. The ZnS04, formed also 
in the oxidized zone, is so readily soluble in water that it is leached from 
the other oxidized compounds and is carried downward. 



4 



PART II 
DESCRIPTIVE MINERALOGY 



CHAPTER III 

INTRODUCTION— THE ELEMENTS 

Of the 1,000 or more distinct minerals recognized by mineralogists 
only a few (some 250) are common. A few are important because they 
constitute ores, others because they are components of rock masses, 
and others simply because of their great abundance. Only a few miner- 
alogists profess acquaintance with more than 500 or 600 minerals. The 
majority are familiar with but 300 or 400, relying for the identification of 
the remainder upon the descriptions of them recorded in mineralogical 
treatises. 

Only the minerals commonly met with and those of economic or of 
special scientific importance are described in this book. They should 
be studied with specimens before one, in order that the relation between 
the descriptions and the objects studied may be forcibly realized. Min- 
eralogy cannot be studied successfully from books alone. It is primarily 
a study of objects and consequently the objects should be at hand for 
inspection. 1 

Mineral Names. — The names of the great majority of minerals end 
in the termination "ite." This is derived from the ancient Greek suffix 
"itis" which was always appended to the names of rocks to signify that 
they are rocks. The first portion of the name, to which the suffix is 
added, either describes some quality or constituent possessed by the 
mineral, refers to some common use to which it has been put, indicates 
the locality from which it was first obtained, or is the name of some 
person intended to be complimented by the mineralogist who first 
described the mineral bearing it. 

1 Collections of the common minerals in specimens large enough for convenient 
study may be secured at small cost from any one of the mineral dealers whose 
addresses may be found in any mineralogical journal. 

36 



INTRODUCTION— THE ELEMENTS 37 

The following examples taken from Dana illustrate some of these 
principles. The mineral hematite (Fe203) is so named because of the red 
color of its powder, chlorite (a complicated silicate), because of its green 
color, siderite (FeCOa), from the Greek word for iron, because it con- 
tains this metal, magnetite (Fe304) after Magnesia in Asia, goethite 
(FeO(OH)) after the poet Goethe. 

The names of a few minerals end in "ine," "ane," "ase," "ote," etc., 
but the present tendency is to have them all end in "ite." Occasionally, 
the same mineral may have two names. This may be due to the fact 
that it was discovered by two mineralogists working at the same time 
in different places, or it may be due to the fact that the mineralogists of 
different countries prefer to follow different precedents set by the old 
mineralogists of their respective nationalities. For example, the min- 
eral (Mg- Fe)2Si04 is called olivine by the Germans and by most English- 
speaking mineralogists, and peridot by the French. The Germans follow 
the German mineralogist Werner, who first used the name olivine in 
1789, while the French follow the French teacher Hatiy, who proposed 
the name peridot in 1801. 

ELEMENTS 

The elements that occur in nature are few in number, and these, 
with rare exceptions, do not occur in great abundance. They may be 
separated into the following groups: the carbon group, the sulphur 
group, the arsenic group, the silver group, and the platinum-iron 
group. Some of these comprise only a single mineral, while others 
comprise six or seven. Only a portion of these are described. 

THE NON-METALS AND METALLOIDS 

CARBON GROUP 
The carbon group embraces several minerals of which one is dia- 
mond, another is an amorphous black substance known as schungite, 
and the other two are apparently but different forms of graphite. 
The element may thereupon be regarded as trimorphous. Diamond 
and graphite are both important. 

Isometric (hextetrahedral) Hexagonal (ditrigonal scalenohedral) 

Diamond Graphite 

Diamond (C) 

The diamond is usually found in distinct crystals or in irregular 
masses, varying in size from a pin's head to a robin's egg. In some 
cases large individual pieces are found but they are exceedingly rare. 



38 



DESCRIPTIVE MINERALOGY 



Fig. ii.— Etch Figures on 
Cubic Face of Diamond 
Crystal. (Afttr Ticker- 



The largest ever found, known as the Cullinan diamond (Fig. 16), 

weighed 3,024! carats or 621 grams, or 1.37 lb., 

and measured 112x64x51 mm - It was cut 

into nine fine gems and a number of smaller 

I ones (Fig. 17). 

I In composition the diamond is pure car- 

I bon, but it is a form of carbon that is not 
ignited and burned at low temperatures. At 
high temperatures, however, especially when 
in the presence of oxygen, it burns freely 
with the production of COa, and, in the case 
of opaque varieties, a little ash. 

Its crystallization is isometric (hextetra- 
hedral class), and the forms on the crystals often appear to be tetra- 
hedrally hemihedral, although the 
etch figures on cubic faces suggest 
hexoctahedral symmetry (Fig. n). 
Octahedrons, tetrahedrons, icositet- 
rahedrons and combinations of these 
forms are common, and in nearly all 
cases the interfacial edges are rounded 
and the crystal faces curved. Some- 
times this curving is so pronounced 
that the individuals are practically 
spheres (Fig. 12). Twins are com- 
mon with 0(i n) as the twinning 
plane (Fig. 13). 

The cleavage of diamond is per- 
fect parallel to the octahedral face. 
This is an important characteristic, as the lapidary makes use of it 
in the preparation of stones for cutting. Its 
fracture is conchoidal. Its specific gravity is 
3.52 and its hardness greater than that of any 
other known substance. Most diamonds are 
dark and opaque, or, at most, translucent, but 
many are found that are transparent and color- 
less or nearly so. Gray, brown, green, yellow, 
blue and red tinted stones are also known, and, 
with the exception of the blue and red diamonds, 
these are more common than the colorless, or 
so-called white stones. The luster of all diamonds is adamantine, and 



2. — Crystal of Diamond with 
Rounded Edges and Faces. (Kraals.) 




r ic. 13.— Octahedron of 
Diamond Twinned 
about O(iii). 



INTRODUCTION— THE ELEMENTS 39 

their index of refraction is very high, «= 2.4024 for red rays, 2.4175 for 
yellow rays, and 2.4513 for blue rays. In consequence of their strong 
dispersion, the reflection of light from the inner surfaces of transparent 
stones is very noticeable, causing them to sparkle brilliantly, with a 
handsome play of colors. It is this latter fact and the great hardness 
of the mineral that make it the most valuable of the gems.- The mineral 
is a nonconductor of electricity. 

Three varieties of the diamond have received distinct names in 
the trade. These are: 

Gem diamonds, which are the transparent stones; 

Bart, or Bortz, gray or black translucent or opaque rounded masses, 
with a rough exterior and the structure of a crystalline aggregate; and 

Carbonado, black, opaque or nearly opaque masses possessing a 
crystalline structure, but no distinct cleavage. 

The only minerals with which diamond is liable to be confused 
are much softer, and, consequently, there is little difficulty in dis- 
tinguishing between them. 

Syntheses. — Small diamonds have been made by fusing in an 
electric furnace metallic iron containing a small quantity of carbon and 
cooling the mass suddenly in a bath of molten lead. They have also 
been made by heating in the electric arc pulverized carbon on a spiral 
of iron wire immersed in hydrogen under a pressure of 3,100 atmospheres. 
A third method, which resulted in the production of tiny octahedrons, 
consisted in melting graphite in olivine, or in a mixture of silicates 
having the composition of the South African " blue ground," with 
the addition of a little metallic aluminium or magnesium. 

Occurrence and Origin. — Diamonds are found (1) in clay, sand 
or gravel deposits or in the rocks formed by the consolidation of these 
substances, where they are associated with gold, platinum, topaz, 
garnet, tourmaline and with other minerals that result from the decom- 
position of granitic rocks, (2) in a basic igneous rock containing frag- 
ments of shale (a consolidated mud) and (3) small diamonds have been 
discovered in meteorites. 

The manner of origin of diamonds has been a subject of contro- 
versy for many years. The most popular theory ascribes the diamonds 
in igneous rocks to the solution of organic matter in the rock magmas 
and the crystallization of the carbon upon cooling. Another theory 
regards the carbon as an original constituent of the magma. The 
diamonds in sand, sandstone, granite, etc., are believed to have been 
transported from their original sources and deposited in river channels 
or on beaches. 



40 DESCRIPTIVE MINERALOGY 

Localities. — The principal localities from which diamonds are obtained 
are the Madras Presidency in India; the Province of Minas-Geraes in 
Brazil; the Island of Borneo; the valleys of the Vaal and Orange 
Rivers, and other places in South Africa, and the valley of the Mazaruni 
River and its tributaries in British Guiana. Recently diamond fields 
have been discovered in New South Wales, Australia, in the valley of 
the Kasai River in the Belgian Kongo, in Arkansas, and in the Tula- 
meen district, British Columbia. 

In the United States a few gem diamonds have been found from 
time to time in Franklin and Rutherford counties in North Carolina; 
in the gold-bearing gravels of California, and in soils and sands in the 
states of Alabama, Virginia, Wisconsin, Indiana, Ohio, Idaho and 
Oregon. A stone (the Dewey diamond) found near Richmond, Virginia, 
a few years ago is valued at $300 or $400. 

The principal source of diamonds and carbonado in Brazil at the 
present time is Bahia, where the mineral occurs in a friable sandstone 
along river courses. The output of this region has decreased so greatly 
in the last few years that although a mass of carbonado weighing 3,073 
carats (the largest mass of diamond material ever found) was obtained 
in 1895, the price of this impure diamond rose from $10.50 per carat 
in 1894 to $36.00 per carat in 1896 and $85.00 per carat for the best 
quality in 19 16. 

The only diamond field of prominence in the United States is that 
which has recently been exploited near Murfreesboro in Arkansas, where 
the conditions are similar to those existing in South Africa. The dia- 
monds occur in a basic igneous rock (peridotite) that cuts through sand- 
stones and quartzites. The peridotite is weathered to a soft earth or 
" ground " in which the diamonds are embedded. Up to the end of 
1914 over 2,000 diamonds had been found, mostly small stones weighing 
in the aggregate 550 carats, valued at about $12,000. One, however, 
weighed 8| carats and another 7! carats. The rough unsorted stones 
are valued at $10 per carat. Three stones that were cut were found 

* 

to be worth from $60 to $175 per carat. The district has not yet been 
sufficiently developed to prove its commercial value. The diamonds 
in British Columbia occur in the same kind of rock as those in Arkansas. 
The few that have thus far been found are too small for any practical 
use. 

In former times the mines of India and Borneo were very produc- 
tive, the famous Golconda district in India for a long period furnishing 
most of the gems to commerce. 

The African mines were opened in 1867. Since this time they 



INTRODUCTION— THE ELEMENTS 41 

have been practically the only producers of gem material in the world. 
It is estimated that the quantity of uncut diamonds yielded by the 
mines near Kimberly alone have amounted in value to the enormous 
sum of $900,000,000. The output of the African mines in 19 13 was 
sold for about $53,000,000, being over 95 per cent of the world's out- 
put of gem material. Of this amount about $9,000,000 worth of stones 
were furnished by German Southwest Africa, the balance by the 
Union of South Africa. The diamonds are found in a peridotite which 
occurs in the form of volcanic necks, or " pipes," cutting carbonaceous 
shales. The igneous rock is much weathered to a soft blue earthy mass 
known as " blue earth." Near the surface where exposed to the action 
of the atmosphere the earth is yellow. The diamonds are scattered 
through the weathered material in quantities amounting to between 
.3 and .6 carat per cubic yard. 

Extraction. — Where the diamond occurs in sand and gravel it is ob- 
tained by washing away the lighter substances. 

In South Africa and Arkansas the mineral is found in a basic volcanic 
rock which weathers rapidly on exposure to the air. The weathered 
rock is mined and spread on a prepared ground to weather. When suf- 
ficiently disintegrated water is added to the mass and the mud thus 
formed is allowed to pass over plates smeared with grease. The dia- 
monds and some of the other materials adhere to the grease, but most 
of the valueless material is carried off by the water. 

Uses. — Transparent diamonds constitute the most valuable gems 
in use. Perfectly white stones, or those possessing decided tints of red, 
rose, green or blue are the most highly prized. They are sold by 
weight, the standard being known as the carat, which, until recently, 
was equivalent to 3.168 grains or 205 milligrams. At present the metric 
carat is in almost universal use. This has a weight of 200 milligrams. 
The price of small stones depends upon their color, brilliancy and size — 
a perfectly white, brilliant, cut stone weighing one carat, being valued 
at about $175.00. As the size increases the value increases in a much 
greater ratio, the price obtained for large stones depending almost solely 
upon the caprice of the purchaser. 

Nearly all the gem diamonds put upon the market are cut before 
being offered for sale. The chief centers of diamond cutting are Ant- 
werp and Amsterdam in the Old World and New York in America. 
The favorite cuts are the brilliant and the rose. For the former only 
octahedral crystals, or those that will yield octahedrons by cleavage, 
are used, for the rose cut distorted octahedrons or twinned crystals. 
In producing the "brilliant" a portion of the top of an octahedron is cut 



42 



DESCRIPTIVE MINERALOGY 



A 



W=W 



A A 







Bom 



Grown Back, ox Parllion 

Step or Trap 






Grown 



gido View 



ParCHon, or Base 

Brilliant 

Fig. 14. — Principal " cuts " of Diamonds. 



off and a small portion of the bottom. On the remainder are cut three 
or four bands of facets running horizontally around the stone (see Fig. 14). 
The "rose" has a flat base surmounted by a pyramidal dome consisting 
of 24 or more facets. In late years the shapes into which diamonds are 
cut have been determined less by the decrees of fashion and more by the 

/v— — 7v desire to save as much ma- 

>n> vtI mO i / On terial as possible, and, conse- 
quently, irregularly shaped cut 
diamonds are much more 
common than formerly (com- 
pare Fig. 17). 

Diamonds are employed 
also as cutting tools. Small 
fragments, or splinters of gem 
quality, are used for cutting 
and polishing diamonds and 
other gems, and small crystals 
with crystal edges for cutting 
glass. Small cleavage pieces 
are utilized in the manufacture of engravers' tools and writing instru- 
ments. Recently diamonds with small holes of from .008 to .0006 of an 
inch drilled in them, have been employed as wire dies. 

Bort is also used as a polishing and cutting material, while carbonado, 
nearly all of which comes from Brazil, is used in the manufacture of 
boring instruments. Diamond drills consist of hollow cylinders of soft 
iron set at their lower edges with 6, 8 or 1 2 black diamonds. By rapid 
revolution of this a "core" may be cut from the hardest rocks. 

Some Famous Diamonds. — The largest diamond ever found — the Cull- 
inan — was picked up at the Premier Mine (Fig. 15) in the Transvaal in 
January, 1905, and was presented to King Edward of England as a birth- 
day gift in 1908. (Figs. 16 and 17.) It weighed about 3,025 carats (about 
1.37 pounds). The next largest was found in June, 1893, at the Jagers- 
fontein mine. It is known as the Excelsior. It weighed in its natural 
state 971 carats and was 3 inches long in its greatest dimension. It was 
valued at $2,000,000. It is said to have been presented by the Presi- 
dent of The Orange Free State to Pope Leo XIII. The third largest 
stone is the Reitz. It is a 640-carat stone found at the same mine during 
the close of 1895. This, though smaller, is said to be handsomer "than the 
Excelsior. The most noted diamond in the world is the Kohinoor, which 
weighed, before cutting, 186 carats. It is now a brilliant of 106 carats, 
belonging to the crown of England. Other famous diamonds are the 



INTRODUCTION— THE ELEMENTS 



FlC. 15, — Premier Diamond Mines in South Africa. (After WUiiem.) 



—The Cullman Diamond. (Natural size.) 



DESCRIPTIVE MINERALOGY 



Fig. 17. — Gems Cut from the Cullinaii Diamond. [Two-fifths nat. size.) 

Orlov, 193 carats, the property of Russia; the Regent or Pitt diamond 
of 137 carats belonging to France; the Green diamond of Dresden, 
weighing 48 carats, and the Blue 
Hope diamond, weighing 44 carats. 
The " Star of the South," found in 
Brazil, weighed 254 carats before 
cutting and 125 afterward. The 
Victoria diamond from one of the 
Kimberly mines weighed 457 carats 
when found. It has been cut to a 
perfect brilliant of 180 carats valued 
at $1,000,000. The Tiffany dia- 
mond (Fig. 18) now owned in New 
York is a double brilliant of a 
golden yellow color weighing 128J 
F.G. l8 .-The Tiffany Diamond. (Nat- carats (25.702 grams) and valued at 
ural size.) (Kindness of Tijjany &■ Co.) $100,000. When it is remembered 
that a five-carat stone is large, tie 
enormous proportions of the above-named gems are better appreciated. 

Graphite (C) 

Graphite, or plumbago, occurs principally in amorphous masses of a 
black, clayey appearance, in. radiated masses, in brilliant lead black 
scales or plates, and occasionally in crystals with a rhombohedral habit. 

Like diamond, graphite consists of carbon. Crystals from Ceylon 
yield: C=79.4o; Ash= 15.50; Volatile matter=5.io. The mineral is 
often impure from admixture with clay, etc. 



INTRODUCTION— THE ELEMENTS 45 

« 

Crystals of the material a*e so rare that their symmetry is still in 
doubt. Their habit is hexagonal (di trigonal scalenohedral class). 
Measurements made on the interfacial angles of crystals from Ticon- 
deroga, New York, gave a : c=i : 1.3859. These possess a rhombo- 
hedral symmetry. All crystals are tabular and nearly all are so distorted 
that the measurements of their interfacial angles cannot be depended 
upon for accuracy. They apparently contain the planes R(ioTi); 
oP(iooo); 00 P2(ii2o), and 2P2(nIi). 

Graphite is black and earthy, or lustrous, according as it is impure 
or pure. It is easily cleavable parallel to the basal plane, and the cleav- 
age laminae are flexible. It is very soft, its hardness being only 1-2, 
its density about 2.25. Its luster is metallic and the mineral is opaque 
even in the thinnest flakes. It is a conductor of electricity. 

Graphite is infusible and noncombustible even at moderately high 
temperatures. Like diamond, however, it may be burned under cer- 
tain conditions at very high temperatures (65o°-7oo°). It is unaffected 
by the common acids and is not acted upon by the atmosphere. 
When, however, it is subjected to the action of strong oxidizing agents, 
such as a warm mixture of potassium chlorate (KCIO3) and fuming 
nitric acid, it changes to a yellow substance known as graphitic acid 
(CiiH40r>). It is thus distinguished from amorphous carbon, like 
schungite and anthracite. Moreover, many forms of graphite, when 
moistened with fuming nitric acid and heated, swell up and send out 
worm-like processes. Those which do not act thus are called graphitite. 
Natural graphite is of both types. 

Its color, softness and infusibility serve to distinguish graphite from 
all other minerals but molybdenite (p. 75). It may be distinguished from 
this mineral by the fact that it contains no sulphur. 

Syntheses. — Crystalline graphite is made on a commercial scale 
by treating anthracite coal or coke containing about 5.75 per cent of 
ash in an electric furnace. It also separates when molten iron con- 
taining dissolved carbon is cooled. 

Occurrence and Origin. — Graphite occurs as thin plates and scales 
in certain igneous rocks, in gneisses, schists and limestones, as large 
scales in coarse granite dikes (pegmatite) and in crystalline limestones, 
and as amorphous masses at the contacts of igneous rocks with carbona- 
ceous rocks. The mineral is also found in veins cutting sedimentary 
and metamorphic rocks. Crystals are found only in limestone. 

The occurrence of graphite in sedimentary and igneous rocks sug- 
gests that it may have been formed in several ways. It is thought 
that the material in limestone and quartz-schist may represent carbo- 



46 DESCRIPTIVE MINERALOGY 

naceous material that was deposited with the sediments and which has 
since been carbonized by heat and pressure. The material in peg- 
matite may be an original constituent of the magma that produced the 
rock, and the graphite may be the product of pneumatolytic processes; 
i.e., it may have been produced by deposits from vapors that accom- 
panied the formation of the pegmatite. If this be true, the mineral 
found in metamorphosed limestone and schist may be of contact origin ; 
i.e., it may have been produced by the migration of gases and solutions 
from igneous rocks into the mass of the surrounding sediments. The 
vein deoosits probably had a similar origin, the mineral having been 
deposited mainly in cracks traversing metamorphic rocks. On the 
other hand, graphite, in some instances, appears to be a direct separa- 
tion from a molten magma. 

Localities. — The principal foreign source of supply for commercial 
graphite is the Island of Ceylon. In the United States the mineral has 
been mined on the southeast side of the Adirondacks in New York; 
in Chester County, Pennsylvania; near Dillon, Montana; at several 
points in Arkansas, Georgia, Alabama and North Carolina; in Wyo- 
ming; in Baraga County, Michigan, and to a small extent in Colorado, 
Nevada, and Wisconsin. It occurs also abundantly at many other 
places. Its chief source in the United States is Graphite, near Lake 
George, New York. 

Preparation. — Graphite is obtained on a commercial scale by grind- 
ing the rock containing it and floating the graphite flakes. 

Uses. — Crude graphite, or plumbago, is used in the manufacture of 
stove and other polishes, and of black paint for metal surfaces, for both 
of which it is especially valuable on account of its noncorroding proper- 
ties. The purified mineral is mixed with clay and made into crucibles 
for use at high temperatures. It is also ground and used in this form 
as a lubricant for heavy machinery, and is compressed into " black lead " 
centers for lead pencils. 

Production. — The quantity of crude graphite mined in the United 
States during 191 2 amounted to 2,445 tons, valued at $207,033, besides 
which there were manufactured 6,448 tons, valued at $830,193. The 
imports were 25,643 tons, valued at $709,337. 

Schungite is a black, amorphous carbon with a hardness of 3-4 
and a sp.gr. of 1.981. It is soluble in a mixture of HNO3 and KCIO3 
without the production of graphitic acid. It occurs in some crystalline 
schists. 



INTRODUCTION— THE ELEMENTS 47 

SULPHUR GROUP 

Sulphur is known in at least six different forms, four of which are 
crystalline. The two best known forms crystallize respectively in the 
orthorhombic (orthorhombic bipyramidal class) and the monoclinic 
(prismatic class) systems. The former separates from solutions of sulphur 
in carbon bisulphide and the latter separates from molten masses. 
Both the orthorhombic and the monoclinic phases are believed to be 
formed by natural processes, but the latter passes over into the former 
upon standing, so that its existence as a mineral cannot be definitely 
proven. Selenium and tellurium, which are also members of the sul- 
phur group, are extremely rare. Tellurium occurs in rhombohedral 
crystals and selenium in mixed crystals of doubtful character with 
sulphur and tellurium. 

Sulphur (S) 

Sulphur occurs in nature as a lemon-colored powder, as spherical or 
globular masses, as stalactites and in crystals. 

Chemically it is pure sulphur, or a mixture of sulphur and clay, 






Fia. 19. Fia. 20. 

Fig. 19. — Sulphur Crystals with P, in (p)\ 3P, 113 (5); P«, on (»), and oP, 

001 (c). 

Fig. 20. — Distorted Crystal of Sulphur. (Forms same as in Fig. 19.) 

bitumen or other impurities. It sometimes contains traces of tellu- 
rium, selenium and arsenic. 

Crystals of sulphur are usually well formed combinations of ortho- 
rhombic bipyramids and domes, with or without basal terminations. 
Their axial ratio =.8108 : 1 : 1.9005. The principal forms observed 
are P(ni), P 66 (101), Poo (on), JP(ii3) and oP(ooi) (Figs. 19 and 
20). The habit of the crystals is usually pyramidal, though crystals 
with a tabular habit are quite common. 

Crystals of sulphur are yellow. Their streak is light lemon yellow. 



48 DESCRIPTIVE MINERALOGY 

The mineral has a resinous luster. Its hardness is only 1.5-2, and 
density about 2.04. Its fracture is conchoidal and cleavage imper- 
fect. It is transparent or translucent, is brittle and is a non- 
conductor of electricity. Its indices of refraction for sodium light 
are a= 1.9579, 0= 2.0377, 7= 2.2452. 

Massive sulphur varies in color from yellow to yellowish brown 
greenish gray, etc., according to the character and amount of impurities 
it contains. Its powder is nearly always crystalline. In mass it pos- 
sesses a lighter color than the crystals or the massive sulphur. 

At a temperature of 114 sulphur melts, and at 270 it ignites, 
burning with a blue flame and evolving fumes of SO2. At about 97 ° 
it passes over into the monoclinic phase. It is insoluble in water and 
acids, but is soluble in oil of turpentine, carbon bisulphide and chlo- 
roform. 

There are few minerals that are apt to be mistaken for sulphur. 
From all of them it may be distinguished by its brittleness and by the 
fact that it melts readily and burns with a nonluminous blue flame. 

Syntheses. — Crystals with the form of the mineral are produced by 
the evaporation of solutions of sulphur in carbon bisulphide, and also 
by sublimation from the fumes of ore roasters. 

Occurrence and Origin. — Sulphur occurs most abundantly in regions 
of active or extinct volcanoes, and in beds associated with limestone 
and gypsum (CaS04-2H20). In volcanic regions it is produced by 
reactions between the gases emitted from the volcanoes, or by the reac- 
tions of these with the oxygen of the air (seep. 18). The deposits in 
gypsum beds may result from reduction of the gypsum by organic 
matter. Sulphur is formed also as a decomposition product of sulphides. 

In Iceland and other districts of hot springs sulphur is often deposited 
in the form of powder as the result of reactions similar to those that 
take place between the gases of volcanoes. These hot springs are always 
connected with dying volcanoes, being frequently but the closing 
stages of their existence. 

Localities. — The localities at which sulphur is known to exist are 
very numerous. Those of commercial importance are Girgenti in Sicily, 
Cadiz in Spain, Japan; and in the United States, at the geysers of the 
Napa Valley, Sonoma County, and at Clear Lake, Lake County, 
California; at Cove Creek, Millard County, Utah; at the mines of the 
Utah Sulphur Company in Beaver County, in the same State; at 
Thermopolis, Wyoming, and at various hot springs in Nevada. The 
mineral occurs also abundantly in the Yellowstone National Park, but 
cannot be placed on the market because of high transportation charges. 



INTRODUCTION— THE ELEMENTS 49 

Its principal occurrence in the United States is at Lake Charles in 
Calcasieu Parish, La., where it impregnates a bed of limestone at 
a depth of from 450 to 1,100 feet. It occurs also abundantly in the 
coastal districts of Texas. Here it is associated with gypsum. 

Extraction. — Sulphur, when mined, is mixed with clay, earth, rock and 
other impurities. Until recently it was purified by piling in heaps and 
igniting. A portion of the sulphur burned and melted the balance, 
which flowed off and was caught. A purer product is obtained by dis- 
tillation. "Flowers of Sulphur" are made in this way. At present 
much of the sulphur is extracted by treating the impregnated rock in 
retorts with steam under a pressure of 60 pounds and at a temperature 
of 144 C. The sulphur melts and flows to the bottom of the retorts 
from which it is drawn off. 

In Louisiana and Texas, superheated steam is forced downward into 
the sulphur-impregnated rocks. This melts the sulphur, which con- 
stitutes about 70 per cent of the rock mass. The melted sulphur is 
forced to the surface and caught in wooden bins. The crude material 
has a guaranteed content of over 99! per cent sulphur. 

Uses. — Sulphur, or brimstone, is used in the manufacture of some 
kinds of matches, in making gunpowder, and in vulcanizing rubber 
to increase its strength and elasticity. It is used extensively in the 
manufacture of sulphuric acid, but is rapidly giving way to pyrite 
for this purpose. It is also utilized for bleaching straw, in the man- 
ufacture of certain pigments, among which is vermilion, and in the 
preparation of certain medicinal compounds. 

Production. — Most of the domestic product is at present from the 
Calcasieu Parish, La., where about 300,000 tons are mined annually. 
New mines have been opened near Thermopolis in Wyoming, in Bra- 
zoria County, Texas, and at Sulphur Springs, Nevada. The total 
amount of the mineral mined in 191 2 was 303,472 tons, valued at $5,256,- 
422. Besides, there were imported about 29,927 tons valued at $583,974, 
most of which came from Japan. Sicily is the largest producer of the 
mineral, extracting about 400,000 tons annually. 

ARSENIC GROUP 

The arsenic group comprehends metallic arsenic, antimony, bismuth 
and (according to some mineralogists), tellurium, besides compounds 
of these metals with each other. They all crystallize in the rhombo- 
hedral division of the hexagonal system (di trigonal scalenohedral class). 
The only members of the group that are at all common are arsenic and 
antimony. 



60 DESCRIPTIVE MINERALOGY 

Arsenic (As) 

Arsenic is rarely found in crystals. It usually occurs massive or in 
botryoidal or globular forms. 

Specimens of the mineral are rarely pure. They usually contain 
some antimony, and traces of iron, silver, bismuth, and other metals. 

The crystals are cubical in habit, with an axial ratio of i : 1.4025. 
The principal forms observed are: oR(oooi), R(ioTi), JR(ioT4), 
— JR(oiT2) and —^(0332). Twins are rare, with — JR(oiT2) the 
twinning plane. 

Arsenic is lead-gray or tin-white on fresh fractures, and dull gray or 
nearly black on surfaces that have been exposed for some time to the 
atmosphere. 

Crystals cleave readily parallel to the base. The fracture of massive 
pieces is uneven. The mineral is brittle. Its hardness is 3.5 and its 
density 5.6-5.7. Its streak is tin- white tarnishing soon to dark gray. 
It is an electrical conductor. 

Arsenic may easily be distinguished from nearly all other minerals, 
except antimony and some of the rarer metals, by the color of its fresh 
surfaces. From these, with the exception of antimony, it is also readily 
distinguished by its action on charcoal before the blowpipe, when it 
volatilizes completely without fusing, at the same time tingeing the 
flame blue and giving rise to dense white fumes of AS2O3, which coat the 
charcoal. The fumes of arsenic possess a very disagreeable and oppres- 
sive odor, while those of antimony have no distinct odor. 

Syntheses. — Arsenic has been obtained in crystals by subliming 
arsenic compounds protected from the air. It has also been obtained in 
the wet way by heating realgar (AS2S3) with sodium bicarbonate at 
3oo°C. 

Occurrence and Origin. — Arsenic often accompanies ores of antimony, 
silver, lead and other metals in veins in crystalline rocks, especially in 
their upper portions, where it was formed by reduction from its com- 
pounds. 

Localities. — The silver mines at Freiberg and other places in Saxony 
afford native arsenic in some quantity. It is found also in the Harz; at 
Zmeov in Siberia; in the silver mines of Chile and elsewhere. 

Within the boundaries of the United States arsenic occurs only in 
small quantity at Haverhill, N. H., at Greenwood, Me., and at a silver 
and gold mine near Leadville, Colo. 

Uses. — Arsenic is used only in the forms of its compounds. The 
native metal occurs too sparingly to be of commercial importance. 



INTRODUCTION— THE ELEMENTS 51 

Most of the arsenic compounds used in commerce are obtained from 
smelter fumes produced by smelting arsenical copper and gold ores. 

Antimony (Sb) 

Antimony is more common than arsenic, which it resembles in many 
respects. It is generally found in lamellar, radial and botryoidal masses, 
though rhombohedral crystals are known. 

Most antimony contains arsenic and traces of silver, lead, iron and 
other metals. 

Its crystals are rhombohedral or tabular in habit, and have an axial 
ratio of a : c=i : 1.3236. The forms observed on them are the same 
as those on arsenic with the addition of ooP2(ii2o), and several 
rhombohedrons. Twinning is often repeated. The cleavage is perfect 
parallel to oP(oooi). 

Antimony exhibits brilliant cleavage surfaces with a tin-white color. 
On exposed surfaces the color is dark gray. The mineral differs from 
arsenic in its greater density which is 6.65-6.72, and in the fact that it 
melts (at 629 ) before volatilizing. Its fumes, moreover, are devoid of 
the garlic odor of arsenic fumes. 

Syntheses. — Crystals of antimony are often obtained from the flues of 
furnaces in which antimonial lead is treated. They have also been 
made by the reduction of antimony compounds by hydrogen at a high 
temperature. 

Occurrence and Localities, — Antimony occurs in lamellar concretions 
in limestone near Sala, Sweden, and at nearly all of the arsenic localities 
mentioned above, especially in veins containing stibnite (Sb2S3) or silver 
ores. It is found also in fairly large quantities in veins near Fredericton, 
York County, New Brunswick, in California and elsewhere. 

Uses. — Although the metal antimony is of considerable importance 
from an economic point of view, being used largely in alloys, the native 
mineral, on account of its rarity, enters little into commerce. Some of 
the antimony used in the arts is produced from its sulphide, stibnite 
(see p. 72). Most of the metal, however, is obtained in the form of a 
lead-antimony alloy in the smelting of lead ores and the refining of pig 
lead. 

Bismuth (Bi) is usually in foliated, granular or arborescent forms, 
and very rarely in rhombohedral crystals, with a : c= 1 : 1.3036. It is 
silver-white with a reddish tinge, is opaque and metallic. Its streak is 
white, its hardness 2-2.5 an d density 9.8. It fuses at 271 °. On charcoal 
it volatilizes and gives a yellow coating. It dissolves in HNO3. When 



52 DESCRIPTIVE MINERALOGY 

this solution is diluted a white precipitate results. The mineral occurs 
in veins with ores of silver, cobalt, lead and zinc. It is of no commercial 
importance. Most of the metal is obtained in the refining of lead. In 1913 
the United States produced 185,000 lbs. and Bolivia about 606,000 lbs. 

Tellurium (Te) usually occurs in prismatic crystals with a tin-white 
color and in finely granular masses in veins of gold and silver ores, 
especially sulphides and tellurides. Its hardness is 2 and density 6.2. 
Before the blowpipe it fuses, colors the flame green, coats the charcoal 
with a white sublimate bordered by red, and yields white fumes. 

The mineral tellurium is of little value as a source of the metal. 
Most of that used in the arts is obtained as a by-product in the elec- 
trolytic refining of copper made from ores containing tellurides and 
from the flue dust of acid chambers and smelting furnaces. The United 
States, in 1913, produced about 10,000 lbs. of tellurium and selenium, 
valued at $35,000. 

THE METALS 

The metallic elements occur as minerals in comparatively small quan- 
tity, most of the metals used in the industries being obtained from their 
compounds. Iron, the most common of all the metals used in com- 
merce, is rare as a mineral, as are also lead and tin. Silver, copper, gold 
and platinum are sufficiently important to be included in our list for 
study. Gold and platinum are known almost exclusively in the metallic 
state. A large portion of the copper produced in this country is also 
native, and some of the silver. 

Silver, copper, lead, gold, mercury and the alloys of gold and mer- 
cury crystallize in distinct crystals belonging to the isometric system 
(hexoctohedral class). Platinum, as usually found, is in small plates 
and grains. Crystals, however, have been described and they, too, are 
isometric. Platinum and iron are separated from the other metals and, 
together with the rare alloys of platinum with iridium and osmium, are 
placed in a distinct group which is dimorphous. The reason for this is 
that platinum, although isometric in crystallization, often contains 
notable traces of iridium, which in its alloy with osmium is hexagonal 
(rhombohedral). Iridium, thus, is dimorphous, hence platinum which 
forms crystals with it and is, therefore, isomorphous with it, must also 
be regarded as dimorphous. The various platinum metals thus com- 
prise an isodimorphous group. Iron is placed in the same group because 
it is so frequently alloyed with platinum. The metals are, therefore, 
divisible into two groups, one of which comprises the metals named at 



INTRODUCTION— THE ELEMENTS 



53 



the beginning of this paragraph and the other consists of the rare metals, 
palladium, platinum, iridium, osmium, iron and their alloys. The 
metal tin, which is tetragonal in its native condition, constitutes a third 
group, but since it is extremely rare it will not be referred to again. 

GOLD GROUP 

This group embraces the native metals, copper, silver, goli, gold- 
amalgam (Au-Hg), silver -amalgam (Ag-Hg), mercury, and leal. All 
crystallize in the isometric system (hexoctahedral class), and all form 
twins, with O(m) the twinning plane. Copper, silver and gold are 
the most important. 

Copper (Ctt) 

Most of the copper of commerce is obtained from one or the other of 

its sulphides. A large portion, however, is 

found native. This occurs in tiny grains and 

flakes, in groups of crystals and in large 

masses of irregular shapes. 

In spite of its softness copper is better 

crystallized than either gold or silver. It is 

true that its crystals are usually flattened and 

Otherwise distorted, but, nevertheless, planes 

can very frequently be detected upon them. 

The principal forms observed arc °o O °o (ioo), 

oo O(no), O(ni), and various tctrahexahedra 

and kositetrahedra. (Figs. 21 and 22.) Some- 
times the crystals are sim- 
ple, in other cases they are 
twinned parallel to O. 
Often they are skeleton 
crystals. Groups of crys- 
tals are very common. 
These possess the arbo- 
rescent forms so frequently 
seen in specimens from 
Keweenaw Point in Mich- 
igan, or are groupings of 
simple forms extended in 
the direction of the cubic 
axes. 
Copper is very ductile and very malleable. Its hardness is only 




Fio. 2i.— Copper Crystal 
with » O, no (d) and 



aa. — Crystal of Copper from Keweenaw Point, 
Mich., with =o0{no) and 202(211). 



54 DESCRIPTIVE MINERALOGY 

2.5-3 an d its density about 8.8. It possesses no cleavage, and its frac- 
ture, like that of the other metals, is hackly. In color it is copper-red 
by reflected light, often tarnishing to a darker shade of red. In very 
thin plates it is translucent with a green color. The metal fuses at 
1083 ° and easily dissolves in acids. It is an excellent conductor of elec- 
tricity. 

Its most characteristic chemical reaction is its solubility in nitric 
acid with the evolution of brownish red fumes of nitrous oxide gas. 

Copper may easily be distinguished from all other substances except 
gold and a few alloys by its malleability and color. It is distinguished 
from gold by the color of its borax bead and by its solubility in nitric 
acid with the production of a blue solution which takes on an intense 
azure color when treated with an excess of ammonia. From the alloys 
that resemble it, copper may be distinguished by its greater softness and 
the fact that it yields no coatings when heated on charcoal, while at the 
same time its solution in nitric acid yields the reaction described above. 

Syntheses. — Copper crystals separate upon cooling solutions of the 
metal in silicate magmas and upon the electrolysis of the aqueous solu- 
tions of its salts. 

Occurrence. — The principal modes of occurrence of the metal are, (1) 
as fine particles disseminated through sandstones and slates, (2) as solid 
masses filling the spaces between the pebbles and boulders making up 
the rock known as conglomerate, (3) in the cavities in old volcanic lavas, 
known as amygdaloid, (4) as crystals or groups of crystals imbedded in 
the calcite of veins, (5) in quartz veins cutting old igneous rocks or 
schists, and (6) associated with the carbonates, malachite and azurite, 
and with its different sulphur compounds, in the weathered zone of 
many veins of copper ores. 

The copper that occurs in the upper portions of veins of copper 
sulphides is plainly of secondary origin. That which occurs in conglom- 
erates and other fragmental rocks and in amygdaloids was evidently 
deposited by water, but whether by ascending magmatic water or by 
descending meteoric water is a matter of doubt. 

Localities. — Native copper is found in Cornwall, England, in Nassau, 
Germany, in Bolivia, Peru, Chile and other South American countries, 
in the Appalachian region of the United States and in the Lake Superior 
region, both on the Canadian and the American sides. 

The most important district in the world producing native copper is 
on Keweenaw Point, in Michigan. The mineral occurs mainly in a bed 
of conglomerate of which it constitutes from 1 to 3 per cent, though it is 
found abundantly also in sandstone and in the amygdaloidal cavities 



INTRODUCTION— THE ELEMENTS 55 

of lavas associated with the conglomerates. Veins of calcite, through 
which groups of bright copper crystals are scattered are also very plentiful 
in many parts of the district. The copper is nearly always mixed with 
silver in visible grains and patches. 

Extraction and Refining. — The rock containing the native metal is 
crushed and the metal is separated from the useless material by wash- 
ing. The concentrates, consisting of the crushed metal mixed with 
particles of rock and other impurities are then refined by smelting 
methods or by electrolysis. 

Uses. — The uses of copper are so many that all of even the important 
uses cannot be mentioned in this place. Both as a metal and in the form 
of its alloys it has been employed for utensils and war implements since 
the earliest times. In recent times one of its principal uses has been for 
the making of telegraph, telephone and trolley wires. It is employed 
extensively in electroplating by all the great newspapers and publishers, 
and is an important constituent of the valuable alloys brass, bronze, 
bell metal and German silver. Its compound, blue vitriol (copper sul- 
phate), is used in galvanic batteries, and its compounds with arsenic 
are utilized as pigments. 

Production. — The world's production of copper amounted to 1,126,- 
000 tons in 191 2, but a large portion of this was obtained from its car- 
bonates and sulphides. The quantity obtained from the native metal is 
unknown. The contribution of the United States to this total was 
about 621,000 tons, valued at about $206,382,500, of which 115,000 tons 
was native copper from the Lake Superior region. The largest single 
mass ever found in the Lake Superior region weighed 420 tons. 

Silver (Ag) 

Silver is usually found in irregular masses, in flat scales, in fibrous 
clusters, and in crystal groups with arborescent or acicular forms. 
Sometimes the crystals are well developed, more frequently they ex- 
hibit only a few distinct faces, but in most cases they are so distorted 
that it is difficult to make out their planes. 

Pure silver is unknown. The mineral as it is usually obtained con- 
tains traces of gold, copper, and often some of the rarer metals, depend- 
ing upon its associations. 

Ideally developed silver crystals are rare. They usually show 
00 O 00 (100), 00 0(i 10), 0(i 11) various tetrahexahedrons and other 
more complicated forms. The majority of the crystals are distorted by 
curved faces and rounded edges, and many of them by flattening or 



56 DESCRIPTIVE MINERALOGY 

elongation. The arborescent groups usually branch at angles of 6o°, 
one of the characteristic angles for groups of isometric crystals. Twins 
are quite common, with O(in) the twinning plane. 

Silver is a white, metallic mineral when its surfaces are clean and 
fresh. As it usually occurs it possesses a gray, black or bluish black 
tarnish which is due to the action of the atmosphere or of solutions. 
The tarnish is commonly either the oxide or the sulphide of silver. 

The mineral has no cleavage. Its fracture is hackly. It is soft 
(hardness 2-3), malleable and ductile, and is an excellent conductor 
of heat and electricity. Its density is about 10.5, varying slightly 
with the character and abundance of its impurities. It fuses at 
960 . 

It is readily soluble in nitric acid forming a solution from which 
a white curdy precipitate of silver chloride is thrown down on the 
addition of any chloride: This precipitate is easily distinguished from 
the corresponding lead chloride by its insolubility in hot water. 

Synthesis. — Crystals bounded by O(in) and 00 O 00 (100) have been 
made by the reduction of silver sulphate solutions, with sulphurous 
acid. 

Occurrence. — Native silver is found in veins with calcite (CaCOs), 
quartz (SiCfe), and other gangues traversing crystalline rocks, like 
granite and various lavas, and also in veins cutting conglomerates 
and other rocks formed from pebbles and sands. It is also disseminated 
in small particles through these rocks. It occurs invisibly disseminated 
in small quantities through many minerals, particularly sulphides, 
and visibly intermingled with native copper. It is abundant in the upper 
weathered zones of many veins of silver-bearing ores, and in the zones 
of secondary enrichment in the same veins. It also occurs in small 
quantity in placers. In general, its origin is similar to that of gold 
(see p. 59). 

Localities. — The localities in which silver is found are too numerous 
to mention. Andreasberg in the Harz has produced many fine crys- 
tallized specimens. The principal deposits now worked are at Cobalt 
in Canada, in Peru, in Idaho, at Butte, Montana, in Arizona and at 
many places in Colorado. On Keweenaw Point, in Michigan, fine 
crystals have been found in the calcite veins cutting the copper-bearing 
rocks, and masses of small size in the native copper so abundant in the 
district. Indeed some of the copper is so rich in silver that the ore 
was in early times mined almost exclusively for its silver content. At 
present the silver is recovered from the copper in the refining process. 
At Cobalt the mineral occurs in well defined veins one inch to one foot 



INTRODUCTION— THE ELEMENTS 57 

or more in width, cutting a series of slightly inclined pre-Cambrian 
beds of fragmental and igneous rocks. The veins contain native silver, 
sulphides and arsenides of cobalt, nickel, iron and copper, calcite and a 
little quartz. Many of the veins are so rich (Fig. 23) that Cobalt has 
become one of the most important camps producing native silver in 
the world. 

Extraction and Refining. — Silver is obtained from placers in small 
quantity by the methods made use of in obtaining gold (see p. 61), 
i.e., by hydraulic mining. When it occurs in quartz veins or in complex 
ores such as constitute the oxidized portion of ore-bodies, the mass 
may be crushed and then treated with quicksilver, which amalgamates 
with the native silver and gold, forming an alloy. Such ores are known 



FlG. 33.— Plate of Silver from Conlanas Mine, Cobalt. 

ins. Weight 37 lbs. {Photo by C, W. Knight.) 

as free milling. The silver is freed from the gold and other metals by 
a refining process. It is separated from native copper by electrolytic 
methods. 

Uses. — Silver Is used in the arts to a very large extent. Jewelry, 
ornaments, tableware and other domestic utensils, chemical apparatus 
and parts of many physical instruments are made of it. It is used also 
in the production of mirrors and in the manufacture of certain compounds 
used in surgery and in photography. Its alloy with copper forms the 
staple coinage of China, Mexico and most of the South American coun- 
tries, and the subsidiary (or small) coinage of most countries. In 
the United States it is used in the coinage of silver dollars and of frac- 
tions of the dollar as small as the dime. The silver coins of the United 
States are nine-tenths silver and one-tenth copper, the latter metal being 
added to give hardness. English coins contain 12$ parts silver to one 



58 DESCRIPTIVE MINERALOGY 

part of copper. In 1912 the world's coinage of silver consumed 161,- 
763,415 oz., with a value after coinage of $171,293,000. 

Production. — The total production of silver in the United States 
during 1912 was over 63,766,000 oz., valued at over $39,197,000, of 
which about $100,000 worth came from placers and $325,000 worth 
from tie copper mines of Michigan. The balance was obtained by 
smelting silver compounds and in the refining of gold, lead, copper and 
zinc ores. The world's production of silver during 1912 was 224,488,- 
000 oz., valued at over $136,937,000, but most of this was obtained 
from the compounds of silver and not from the native metal. The 
proportion obtained from the mineral is not definitely known; but the 
production of Canada was more than 30,243,000 oz., valued at 
$17,672,000 and nearly all of this came from Cobalt, where the ore is 
native silver. 

Gold (Au) 

A large portion of the gold of the world has been obtained in the 
form of native metal. The greater portion of the metal is so very finely 
disseminated through other minerals that no sign of its presence can be 
detected even with high powers of the microscope. Although present 
in such minute quantities it is very widely spread, many rocks con- 
taining it in appreciable quantities. Its visible grains, as usually found, 
are little rounded particles or thin plates or 
scales mixed with sand or gravel, or tiny 
irregular masses scattered through white vein- 
quartz. 

Native gold rarely occurs in well formed 
crystals. The metal is so soft that its crystals 
are battered and distorted by very slight 
pressure. Occasionally well developed crys- 
tals, bounded by octahedral, dodecahedral 
Fie. 14 -Octahedwl Skde- and complicated i cos i t etrahedral and tetra- 
ton Crystal of Gold with , , , , , . , , 

Etched Faces hexahedral faces are met with, but usually 

the crystals are elongated or flattened. Skele- 
ton crystals (Fig. 24) and groups of crystals are more frequently found 
than are simple crystals. Twins are common, with 0(m) the twin- 
ning plane. 

As found in nature, gold is frequently alloyed with silver and it 
often contains traces of iron and copper and sometimes small quanti- 
ties of the rarer metals. 

Gold containing but a trace of silver up to 16 per cent of this metal 



INTRODUCTION— THE ELEMENTS 59 

is known simply as gold. When the percentage of silver present is 
larger it is said to be argentiferous. When the percentage reaches 
20 per cent cr above the alloy is called clectrun. Palladium, rhodium 
and bismuth gold are alloys of the last-named metal with the rare metals 
palladium or rhodium or with the more common bismuth. 

The color of the different varieties of the mineral varies from pinkish 
silver-white to almost copper-red. Pure gold is golden yellow. With 
increase cf silver it becomes lighter in color and with increase in copper, 
darker. The rich red-yellow of much of the gold used in the arts is due 
to the admixture of copper. In very thin plates or leaves (.001 mm.) 
gold is translucent with a blue or green tint. 

Gold is soft, malleable and ductile. Its luster is, of course, metallic 
and its streak, yellow. When pure its density is 19.43, its hardness 
between 2 and 3, and its fusing point 1062 . The metal is insoluble in 
most acids, but it is readily dissolved in a mixture of nitric and hydro- 
chloric acids (aqua regia). It is not acted upon by water or the atmos- 
phere. Its negative properties distinguish it from the other substances 
which it resembles in appearance. It is a good conductor of electricity. 

Syntheses. — Crystals of gold have been obtained by heating a solu- 
tion of AuCfe in amy! alcohol, and by treating an acid solution of the 
same compound with formaldehyde. 

Occurrence. — Native gold is found in the quartz of veins cutting 
through granite and schistose rocks, or in the gravels and sands of rivers 
whose channels cut through these, and in the sands of beaches bordering 
gold-producing districts. It is sometimes found in the compacted 
gravels of old river beds, in a rock known as conglomerate, and in sand- 
stones. It is also present in small quantities in many volcanic rocks, 
and is disseminated through pyrite (FeS2) and some other sulphur com- 
pounds and their oxidation products. 

The gold in quartz veins occurs as grains and scales scattered through 
quartz irregularly, often in such small particles as to be invisible to the 
naked eye, or as aggregates of crystals in cavities in the quartz. Pyrite 
is nearly always associated with the gold. On surfaces exposed to the 
weather the pyrite rusts out and stains the quartz, leaving it cavernous 
or cellular. 

Most of the world's supply of gold has come from placers. These 
are accumulations of sand or gravel in the beds of old river courses. 
The sands of modern streams often contain considerable quantities of 
gold. Many of the older streams were much larger than the modern 
ones draining the same regions and, consequently, their beds contain 
more gold. This was originally brought down from the mountains or 



60 DESCRIPTIVE MINERALOGY 

highlands in which the streams had their sources. The sands and 
gravels were rolled along the streams' bottoms and their greater portion 
was swept away by the currents into the lowlands. The gold, however, 
being much heavier than the sands and pebble grains, merely rolled 
along the bottoms, dropping here and there into depressions from which 
it could not be removed. As the streams contracted in volume the gold 
grains were covered by detritus, or perhaps a lava stream flowing along 
the old river channel buried them. These buried river channels with 
their stores of sands, gravels and gold constitute the placers. With the 
gold are often associated zircon crystals, garnets, diamonds, topazes 
and other gem minerals. Alluvial gold is usually in flattened scales or 
in aggregates of scales forming nuggets. Some of the nuggets are so 
large, 190 pounds or more in weight, that it is thought they may have 
been formed by some process of cementation after they were transported 
to their present positions. 

The gold-quartz veins are usually closely associated with igneous 
rocks, but the veins themselves may cut through sedimentary beds or 
crystalline schists. The veins are supposed to have been filled from 
below by ascending solutions. Metallic gold is also present in the oxi- 
dized zones of many veins of gold-bearing sulphides and in the zones of 
secondary enrichment. At the surface the iron sulphides are oxidized 
into sulphates, leaving part of the gold in the metallic state and dissolv- 
ing another part which is carried downward and precipitated. 

Principal Localities, — Vein gold occurs in greater or less quantity in 
all districts of crystalline rocks. It has been obtained in large quantity 
along the eastern flanks of the Ural Mountains, this having been the 
most productive region in the world between the years 1819 and 1849. 
It has been obtained also from the Altai Mountains in Siberia, from the 
mountains in southeastern Brazil, from the highlands of many of the 
Central and South American countries, and from the western portion of 
the United States, more particularly from the western slopes of the Sierra 
Nevada Mountains and the higher portions of the Rocky Mountains. 
In recent years auriferous quartz veins have been worked at various 
points in Alaska, at Porcupine, Ontario, and other points in Canada. 

The great placer mines of the world are in California, Australia and 
Alaska. In Australia the principal gold mines are situated in the streams 
rising in the mountains of New South Wales and their extension into 
Victoria. The valleys of the Yukon and other rivers in Alaska have 
lately attracted much attention, and in the past few years the beach 
sands off Nome have yielded much of the metal. 

The most important production at present is from South Africa 



INTRODUCTION— THE ELEMENTS 61 

where the metal occurs in an old conglomerate. In the opinion of some 
geologists this is an old beach deposit; in the opinion of others the gold 
was introduced into the conglomerate long after it had consolidated. 

The sands of many streams in Europe and in the eastern United 
States have for many years been "panned" or washed for gold. The 
South Atlantic States, before the discovery of gold in California, in 
1849, yielded annually about a million dollars' worth of the precious 
metal. All of it was obtained by working the gravels and sands of small 
rivers and rivulets. Many of these streams have been worked over 
several times at a profit and the mining continues to the present day. 
Small quantities of gold have also been obtained from streams in Maine, 
New Hampshire, Maryland and other Atlantic coast states. 

Extraction and Refining. — Gold is extracted from alluvial sands 
and from placers by washing in pans or troughs. The sand, gravel 
and foreign particles are carried away by currents of water and 
the gold settles down with other heavy minerals to the bottom of the 
shallow pans used in hand washing, or into compartments prepared for 
it in troughs when the processes are on a larger scale. It is after- 
ward collected by shaking it with mercury or quicksilver, in which it 
dissolves. The quicksilver is finally driven off by heat and the gold 
left behind. Auriferous beach sands and many lake, swamp and river 
sands are dredged and the sand thus raised is treated by similar methods. 
Sands containing as low as 15 cents' worth of metal per cubic yard can 
be worked profitably under favorable conditions. 

Where the gold occurs free (not disseminated through sulphides) 
in quartz the rock is crushed to a fine pulp with water and the mixture 
allowed to flow over copper plates coated with quicksilver. The gold 
unites with the quicksilver and forms an alloy from which the mercury 
is driven off by heat. The process of forming alloys of silver or gold 
with mercury is known as amalgamation. 

When the gold is disseminated through sulphides, these are concen- 
trated, i.e., freed from the gangue material by washing, and then 
roasted. This liberates the gold which is collected by amalgamation, 
or is dissolved by chlorine or cyanide solutions and then precipitated. 

Uses. — Gold, like silver, is used in the manufacture of jewelry and or- 
naments, in the manufacture of gold leaf for gilding and in the produc- 
tion of valuable pigments such as the "purple of Cassius." It also con- 
stitutes the principle medium for coinage in nearly all of the most 
important countries of the world. The gold coins of the United States 
contain 900 parts gold in 1,000. Those of Great Britain contain 916.66 
parts, the remaining parts consisting of copper and silver. The total 



62 DESCRIPTIVE MINERALOGY 

gold coinage of the United States mints from the time of their organi- 
zation to the end of the year 191 2 amounted to $2,765,900,000. The 
gold coined in the world's mints in 191 2 amounted in value to $360,- 
671,382, and that consumed in arts and industries to $174,100,000. 
Jewelers estimate the fineness of gold in carats, 24-carat gold being pure. 
Eigh teen-carat gold is gold containing 18 parts of pure gold and 6 parts 
of some less valuable metal, usually copper. The copper is added to 
increase the hardness of the metal and to give it a darker color. The 
gold used most in jewelry is 14 or 1 2 carats fine. 

Production. — The total value of the gold product of the United 
States during 191 2 was $93,451,000. Of this the following states and 
territories were the largest producers: 

Alaska $17,198,000 Nevada $13,576,000 

California 20,008,000 South Dakota 7,823,000 

Colorado 18,741,000 Utah 4,312,000 

Of the total product, placers yielded gold valued at $23,019,633, and 
quartz veins, metal valued at $62,112,000. The balance of the gold was 
obtained from ores mined mainly for other metals, and in these it is 
probably not in the metallic state. Moreover, some of the ore in quartz 
veins is a gold telluride, but by far the greater portion of the product 
from the quartz veins and placers was furnished by the native metal. 

The world's yield of the precious metal in 191 2 was valued at $466,- 
136,100. The principal producing countries and the value of the gold 
produced by each were : 

South Africa $211,850,600 Mexico $24,450,000 

United States 93>45i,5QO India 11,055,700 

Australasia 54,509,400 Canada 12,648,800 

Russia 22,199,000 Japan 4,467,000 

Lead occurs very rarely as octahedral or dodecahedral crystals, 
in thin plates and as small nodular masses in districts containing man- 
ganese and lead ores and also in a few placers. It usually contains 
small quantities of silver and antimony. The native metal has the 
same properties as the commercial metal. Its hardness is 1.5 and 
density 11.3. It melts at about 33. 5 °. 

The mineral is of no commercial importance. The metal is obtained 
from galena and other lead compounds. 

Mercury occurs as small liquid globules in veins of cinnabar (HgS) 
from which it has probably been reduced by organic substances, and ig 



INTRODUCTION— THE ELEMENTS 63 

the rocks traversed by these veins. The native metal possesses the 
same properties as the commercial metal. It solidifies at — 39 °, when 
it crystallizes in octahedrons having a cubic cleavage. Its density is 
13.6. Its boiling-point is 350 . 

The commercial metal is obtained from cinnabar (p. 98). 

Amalgam (Ag • Hg) is found in dodecahedral crystals in a few places, 
associated with mercury and silver ores. It occurs also as embedded 
grains, in dense masses and as coatings on other minerals. It is silver- 
white and opaque and gives a distinct silver streak when rubbed on 
copper. Its hardness is about 3 and its density 13.9. When heated 
in the closed tube it yields a sublimate of mercury and a residue of 
silver. On charcoal the mercury volatilizes, leaving a silver globule, 
soluble in nitric acid. 

PLATINUM-IRON GROUP 

The platinum-iron group of minerals may be divided into the plati- 
num and the iron subgroups. The latter comprises only iron and nickel- 
irony both of which are extremely rare; and the former, the metals 
platinum, iridium, osmium, ruthenium, rhodium, and palladium. The 
platinum metals probably constitute an isodimorphous group since 
they occur together in alloys, some of which are isometric and others 
hexagonal (rhombohedral). Platinum is the only member of the group 
of economic importance. 

Platinum (Pt) 

Platinum occurs but rarely in crystals. It is almost universally 
found as granular plates associated with gold in the sands of streams 
and rivers, and rarely as tiny grains or flakes in certain very basic 
igneous rocks. 

As found in nature the metal always contains iron, iridium, rhodium, 
palladium and often other metals. A specimen from California yielded: 

Cu Ir.Os Sand Total 
1.40 1. 10 2.95 101.15 

Though the metal occurs usually in grains and plates, nevertheless 
its crystals are sometimes found. On them cubic faces are the most 
prominent ones, though the octahedrons, the dodecahedrons and 
tetrahexahedrons have also been identified. Like the crystals of silver 
and gold, those of platinum are frequently distorted. 



Pt 


Au 


Fe 


It 


Rh 


Pd 


85.50 


.80 


6-75 


1 05 


1. 00 


.60 



64 DESCRIPTIVE MINERALOGY 

The color of platinum is a little more gray than that of silver. Its 
streak is also gray. Its hardness is 4-4.5 an ^ density 14 to 19. Pure 
platinum has a density of 21.5. It is malleable and ductile, a good 
conductor" of electricity, and it is infusible before the blowpipe except 
in very fine wire. It is not dissolved by any single acid, though soluble, 
like gold, in aqua regia. Its melting temperature is 1755 . 

Syntheses. — Crystals have been obtained by cooling siliceous mag- 
mas containing the metal, and by dissolving the metal in saltpeter and 
cooling the mixture. 

Occurrence. — Platinum is found in the sands of rivers or beaches 
and in placer deposits in which it occurs in flattened scales or in 
small grains. Nuggets of considerable size are sometimes met with, 
the largest known weighing about i8f kilos. It is present also in 
small quantity in certain very basic igneous rocks, like peridotite. 

Localities. — It occurs in nearly all auriferous placer districts and 
in small quantities in the sands of many rivers, among them the Ivalo 
in Lapland, the Rhine, the rivers of British Columbia, and of the Pacific 
States. It is more abundant in the Natoos Mountains in Borneo, on 
the east flanks of the Ural Mountains in Siberia, in the placer of an 
old river in New South Wales, Australia, and the sands of rivers of 
the Pacific side of Colombia. It is nearly always associated with 
chromite (p. 200). A recent discovery which may prove to be of con- 
siderable importance is near Goodsprings, Nev., where platinum is in 
the free state associated with gold in a siliceous ore. 

The native metal is probably an original constituent of some peri- 
dotites (basic igneous rocks). Its presence in placers is due to the 
disintegration of these rocks by atmospheric agencies. 

Extraction and Refining. — The metal is separated from the sand 
with which it is mixed by washing and hand picking. The metallic 
powder is then refined by chemical methods. 

Uses. — On account of its infusibility and its power to resist the cor- 
rosion of most chemicals the metal is used extensively for crucibles 
and other apparatus necessary to the work of the chemist. It is also 
used by dentists and by the manufacturers of incandescent electric 
lamps. It is an important metal in the manufacture of physical and 
certain surgical instruments, and was formerly used by Russia for coin- 
age. The most important use of the metal in the industries is in the 
manufacture of sulphuric acid. Sulphur dioxide (SO2) and steam when 
mixed and passed over the finely divided metal unite and form H2SO4. 
: More than half of the acid made at present is manufactured by this 
: process. 



INTRODUCTION— THE ELEMENTS 65 

Production. — Most of the platinum of the world is obtained from 
placers in the Urals in Russia. A small quantity is washed from the 
sands of gold placers in Colombia, Oregon and California, and an even 
smaller quantity is obtained during the refining of copper from the ores 
of certain mines. The total production of the world in 191 2 was 
314,751 oz. The output for Russia in this year was about 300,000 oz., 
of Colombia about 12,000 oz., and of the United States 721 oz. (equiv- 
alent to 505 oz. of the refined metal, valued at $22,750). In addition, 
about 1,300 oz. were obtained in the refining of copper bullion imported 
from Sudbury, Ont., and in the treatment of concentrates from the 
New Rambler Mine, Wyoming. Of this about 500 oz. were produced 



Fig. 35. — Iron Meteorite (Siderite) from Canyon Diablo, Arizona. Weight 365 

lbs. {Field Columbian Museum.) 

from domestic ores. The importations into the United States for the 
same year were about 125,000 oz., valued at $4,500,000. 

Platinum-iron, or iron-platinum fPt-Fe), contains from 10 per cent 
to 19 per cent Fe. It is usually dark gray or black and is magnetic. It 
is found with platinum in sands of the rivers in the Urals. Its crystals 
are isometric. 

Iron (Fe) occurs in small grains and large masses in the basalt at 
Ovifak, Disko Island, W. Greenland, and at a few other points in Green- 
land, and alloys consisting mainly of iron are found in the sands of some 
rivers in New Zealand, Oregon and elsewhere. The native metal always 
contains some nickel. The most common occurrence of iron, however, is 
in meteorites (Fig. 25). In these bodies also it is alloyed with Ni. When 



66 DESCRIPTIVE MINERALOGY 

polished and treated with nitric acid, surfaces of meteoric iron exhibit 

series of lines (Widmanstatten figures), that are the edges of plates of 
different composition (Fig. 26). These are so arranged as to indicate 
that the substance crystallizes in the isometric system. 

Iridium (Ir-Pt) and pUtin-iridium (Pt-Ir) are alloys of iridium and 
platinum found as silver-white grains with a yellowish tinge, associated 
with platinum in the sands of rivers in the Urals, Burmah and Brazil. 
Their hardness is 6 to 7, and density 22.7. The mineral is isometric 

and its fusing point is between 2iso -225o". 



FlC. 16. — WidmanslStten Figures on Etched Surface of Meteorite from Toluca, 
Mexico. (One-half natural size.) (Field Columbian Museum.) 

Palladium (Pd) is usually alloyed with a little Pb and Ir. It is 
found in small octahedrons and cubes and also in radially fibrous grains 
in the platinum sands of Brazil, the Urals and a few other places. It is 
whitish steel-gray in color, has a hardness of 4 to 5 and a density of 
11. 3 to 11. 8. It fuses at about 1549°. Its crystallization is isometric. 
About 2,300 oz. of the metal were produced in the United States during 
1912, but all of it was obtained during the refining of bullion. The 
imports were 4,967 oz., valued at $213,397. 

Allopalladium (Pd) is probably a dimorph of palladium. It is found 
in six-sided plates that are probably rhombohedral, intimately asso- 
ciated with gold, at Tilkerode, Harz. 



INTRODUCTION— THE ELEMENTS 67 

Osmiridium (Os • Ir) and iridosmine (Ir • Os) are found in crystals and 
flattened grains and plates that are apparently rhombohedral. They 
consist of Ir and Os in different proportions, often with the addition 
of rhodium and ruthenium. Osmiridium is tin-white and iridosmine 
steel-gray. Their hardness is 6 to 7 and density 19 to 21. When heated 
with KNO3 and KOH, both yield the distinctive chlorine-like odor of 
osmium oxide (OSO4) and a green mass, which, when boiled with 
water, leaves a residue of blue iridium oxide. Both are insoluble in 
concentrated aqua regia. They occur with platinum in the sands of 
rivers in Colombia, Brazil, California, the Urals, Borneo, New South 
Wales, and a few other places. They are distinguished from platinum 
by greater hardness, light color and insolubility in strong aqua regia. 

The world's product of refined iridium is about 5,000 oz., of which 
the United States furnishes about 500 oz. Its value is $63 per oz. 
Imports into the United States during 191 1 were 3,905 oz., valued at 
$210,616. The sources of the metal are native iridium, osmiridium, 
platinum, copper ore and bullion. The metal is obtained from the last 
two sources in the refining process. 



> 



CHAPTER IV 

THE SULPHIDES, TELLURITES, SELENIDES, ARSENIDES AND 

ANTIMONIDES 

The sulphides are combinations of the metals, or of elements acting 
like bases, with sulphur. They may all be regarded as derivatives of 
hydrogen sulphide (H2S) by the replacement of the hydrogen by some 
metallic element. The tellurides are the corresponding compounds of 
H2Te, and the selenides of H2SC 

With the same group are also placed the arsenides and the anti- 
monides, derivatives of H3AS and HaSb, because arsenic and antimony 
so often replace in part the sulphur of the sulphides, forming with these 
isomorphous mixtures. 

The minerals described in this volume may be separated into the 
following groups and subgroups: 

I. The sulphides, tellurides and selenides of the metalloids arsenic, 
antimony, bismuth and molybdenum. 

II. The sulphides, tellurides, selenides, arsenides and antimonides 
of the metals. 

(a) The monosulphides, etc. (Derivatives of H2S, EfeSe, H2Te, 

H3AS, HsSb.) 

(b) The disulphides, etc. (Derivatives of 2H2S, 2H2Te, 2H3AS, 

2HaSb.) 

All sulphur compounds when mixed with dry sodium carbonate 
(Na2C03) and heated to fusion on charcoal yield a mass containing 
sodium sulphide (Na2S). If the mass is removed from the charcoal, 
placed on a bright piece of silver and moistened with a drop or two of 
water or hydrochloric acid, the solution formed will stain the silver a 
dark brown or black color (Ag2S), which will not rub off. The sulphides 
yield the sulphur reaction when heated with the carbonate on platinum 
foil; the sulphates only when charcoal or some other reducing agent is 
added to the mixture before fusing. Moreover, the sulphides yield 
sulphureted hydrogen when heated with hydrochloric acid, while the 
sulphates do not. These tests are extremely delicate. By the aid of 

68 



SULPHIDES, TELLURIDES, ETC. 69 

the first one the sulphur in any compound may be detected. By the 
aid of the others the sulphates may be distinguished from the 
sulphides. 

The selenides are recognized by the strong odor evolved when heated 
before the blowpipe. Selenates and selenites give their odor only after 
reduction with Na2C03. 

The tellurides, when warmed with concentrated H2SO4, dissolve and 
yield a carmine solution from which water precipitates a black gray 
powder of tellurium. 

All substances containing arsenic and antimony yield dense white 
fumes when heated on charcoal in the oxidizing flame. The fumes of 
arsenic possess a characteristic odor while those of antimony are odorless. 
When heated in the open tube, arsenides and compounds with sulphur 
and arsenic yield a very volatile sublimate composed of tiny white crys- 
tals (AS2O3). The corresponding sublimate for antimonides and for 
compounds with antimony and sulphur is nonvolatile, or difficultly 
volatile, and apparently amorphous. It is usually found on the under 
side of the tube. 

THE SULPHIDES, SELENIDES AND TELLURIDES OF 

THE METALLOIDS 

The sulphides of the metalloids include compounds of sulphur with 
arsenic, antimony, bismuth and molybdenum and a selenide and several 
tellurides of bismuth. Only the sulphides are of importance. One, 
stibnite (Sb2S3), is utilized as a source of antimony. 

Realgar (As 2 S 2 ) 

Realgar occurs as a bright red incrustation on other substances, 
as compact and granular masses and as crystals implanted on other 
minerals. It is usually associated with the bright yellow orpiment 

(p. 7i)- 

Absolutely pure realgar should have the following composition: 

As, 70.1 per cent, S, 29.9 per cent. The mineral, however, usually 

contains a small amount of impurities. It may be looked upon as a 

derivative of H2S in which the hydrogen of two molecules is replaced 

by two arsenic atoms, thus: 

H 2 S As=S 

yielding | 
H 2 S As=S. 



70 



DESCRIPTIVE MINERALOGY 




(c); PSb, on (q) and P, 
In ($). 



Crystals of realgar are usually short and prismatic in habit. They 
are monoclinic (prismatic class) with an axial ratio a : b : c : =1.44". 
1 : .973 and 0=66° 5'. The characteristic prismatic faces are 
(w)ooP(no) and (/) 00 P2(2io). These with (b) 00 P So (010) con- 
stitute the prismatic zone. The terminations are (r)£Pob(oi2) or 
(q) Poo (on) in combination with the basal plane (c) oP(ooi), the 
orthodome (x) (Toi), and one or more of several pyramids. (See Fig. 

27.) The crystals are usually small and are 
striated vertically. Prismatic angle no A 1 "io 
= 105 34'. 

The mineral possesses a distinct cleavage 
parallel to (b) 00 P 00 and (/) 00 Pi. It is 
sectile, soft (H= 1.5-2), resinous in luster and 
aurora-red or orange in color. Its streak is a 
lighter shade, but with the mineral are fre- 
quently intermingled small quantities of orpi- 
Fig. 27.— Realgar Crystal, ment which impart to its streak a distinct 
ooP,Mo(m);«P2 ? 2io(/); yellow tinge. Its density is 3.56. In thin 
oop 5b, 010 («; oP, 001 splinters it is often translucent or trans- 
parent, and strongly pleochroic in red and 
yellow tints, but in masses it is opaque. Its 
indices of refraction are not known with accuracy, but its double re- 
fraction is strong (.030). It is a nonconductor of electricity. 

When heated on charcoal before the blowpipe realgar catches fire 
and burns with a light blue flame, at the same time giving off dense 
clouds of arsenic fumes and the odor of burning sulphur (SO2). When 
heated in a closed tube it melts, volatilizes and yields a transparent 
red sublimate in the cold parts of the tube. 

Its bright red color and its reaction for sulphur distinguish realgar 
from all other minerals but cinnabar, the sulphide of mercury (p. 98). 
It may easily be distinguished from cinnabar by its softness, its low 
specific gravity and the arsenic fumes which it yields when heated on 
charcoal. 

On exposure to the air and to light realgar oxidizes, yielding orpi- 
ment (AS2S3) and arsenolite (AS2O3). 

Syntheses. — Realgar is often produced in the flues of furnaces in 
which ores containing sulphur and arsenic are roasted. Crystals have 
also been produced by heating to 150 a mixture of AsS with an excess 
of sulphur in a solution of bicarbonate of soda sealed in a glass tube. 

Occurrence Localities and Origin. — Realgar occurs in masses asso 
ciated with orpiment and in grains scattered through it at all places 



SULPHIDES, TELLURIDES, ETC. 71 

where the latter mineral is found. It also occurs associated with silver 
and lead ores in many places. It is found in crystals implanted on 
quartz and on the walls of cavities in lavas. It is also occasionally 
a deposit from hot springs. In the United States it forms seams in a 
sandy clay in Iron Co., Utah. Its crystals are found in calcite in San 
Bernardino and Trinity Counties, California, and with orpiment it is 
deposited as a powder by the hot water of the Norris Geyser basin in the 
Yellowstone National Park. 

In most cases it is a product of the interaction of arsenic and sul- 
phur vapors. 

Uses. — The native realgar occurs in too small a quantity to be of 
commercial importance. An artificial realgar is employed in tanning 
and in the manufacture of " white-fire." 

Orpiment (As 2 S 3 ) 

Orpiment, though more abundant than realgar, is not a common 
mineral. It is usually found in foliated or columnar masses with a 
bright yellow color. Its name — a contraction from the Latin auri- 
pigmentum, meaning golden paint — refers to this color. 

The pure mineral contains 39 per cent of sulphur and 61 per cent 
of arsenic, corresponding to the formula AS2S3. It thus contains 
about 9 per cent more sulphur than does realgar. 

The monoclinic orpiment crystals have the symmetry of the pris- 
matic class. Their axial ratio is .596 : 1 : .665 with £=89° 19'. Though 
always small they are distinctly prismatic with an orthorhombic habit. 
Their predominant faces are the ortho and clino pinacoids, several 
prisms and the orthodome. 

The cleavage of orpiment is so perfect parallel to 00 P 00 (010) that 
even from large masses of the mineral distinct foliae may be split. 
These are flexible but not elastic. The mineral, like many other 
flexible minerals, is sectile. Its luster is pearly on cleavage faces, 
which are always vertically striated, and is resinous on other surfaces. 
The color of pure orpiment is lemon-yellow; it shades into orange 
when the mineral is impure through the admixture of realgar. Its 
streak is always of some lighter shade than that of the mineral. Its 
hardness is 1.5-2 and its density about 3.4. In small pieces orpiment 
is translucent and possesses an orange and greenish yellow pleochroism. 
When heated to ioo° it becomes red and assumes the pleochroism of 
realgar. It, however, resumes its characteristic color and pleochroism 
upon cooling. When heated to 150 the change is permanent. The 
mineral is a nonconductor of electricity. 



72 



DESCRIPTIVE MINERALOGY 



The chemical properties of orpiment are the same as those described 
for realgar, except that the sublimate in the closed tube is yellow instead 
of red. 

Synthesis. — Orpiment is produced in large pleochroic crystals by 
treatment of arsenic acid with H2S under high pressure. 

Occurrence, Localities and Origin. — Orpiment occurs in the same 
forms and in the same places as does realgar. Small specks of it occur 
on arsenical iron at Edenville, N. Y. It is also found in the deposits 
of Steamboat Springs, Nevada. The origin of orpiment is similar 
to that of realgar. It is also formed by the oxidation of this mineral. 

Uses. — Native orpiment mixed with water and slaked lime is used 
in the East as a wash for removing hair. It is also employed as a pig- 
ment in dyeing. Most of the AS2S3 of commerce is a manufactured 
product. 

STIBNITE GROUP (R a Q 8 ) 

The stibnite group of sulphides contains several isomorphous 
compounds, of which we shall consider only two, viz., stibnite (Sb2Ss) 
and bismuthinite (Bi2S3). The general formula of the group is R2Q3, 
in which R stands for Sb or Bi and Q for S or Se. The group is 
orthorhombic (bipyramidal class). All the members have a distinct 
cleavage parallel to the brachypinacoid which yields flexible laminae. 




m 



m 



Stibnite (Sb 2 S 3 ) 

Stibnite is the commonest and the most important ore of anti- 
mony. It is found in acicular and prismatic crys- 
tals, in radiating groups of crystals and in 
fibrous masses. 

Chemically, stibnite is the antimony trisul- 
phide, Sb2S3, composed of Sb, 71.4 per cent 
and S, 28.6 per cent. As found, however, it 
usually contains small quantities of iron and often 
traces of silver and gold. 

Crystals of stibnite are often very compli- 
cated. They are orthorhombic with an axial ratio 
.9926 : 1 : 1. 01 79 and a columnar or acicular 
habit. The most important forms in the pris- 
matic zone are 00 P(no) and 00 P 06 (010). The 
prisms are often acutely terminated by P(in), ^4(431) and 6P2(36i), 
or bluntly terminated by £P(ii3), (Fig. 28). Sometimes the crystals 
are rendered very complicated by the great number of their terminal 



Fig. 28.— Stibnite Crys- 
tal. 00 P, 110 (w); 
00 Poo , 010 (6); 2P2, 
121 (t>) andP, in (p). 



SULPHIDES, TELLURIDES, ETC. 73 

planes. Dana figures a crystal from Japan that possesses a termina- 
tion of 84 planes, no a 1 10=89° 34'. 

Many of the crystals of this mineral, more particularly those with 
an acicular habit, are curved, bent or twisted. Nearly all, whether 
curved or straight, are longitudinally striated. 

The cleavage of stibnite is very perfect parallel to 00 P 06 (010), 
leaving striated surfaces. The mineral is soft (H=2) and slightly 
sectile. Its density is about 4.5. Its color is lead-gray and its streak 
a little darker. In very thin splinters it is translucent in red or yellow 
tints. In these the indices of refraction for yellow light have been 
determined to be, 0=4.303 and 7 = 3.194. Surfaces that are exposed 
to the air are often coated with a black or an iridescent tarnish. The 
luster of the mineral is metallic. It is a nonconductor of electricity. 

Stibnite fuses very easily, thin splinters being melted even in the 
flame of a candle. When heated on charcoal the mineral yields anti- 
mony and sulphurous fumes, the former of which coat the charcoal white 
in the vicinity of the assay. When heated in the open tube SO2 is 
evolved and a white sublimate of SD2O3 is deposited on the cool walls of 
the tube. In the closed tube the mineral gives a faint ring of sulphur 
and a red coating of antimony oxysulphide. It is soluble in nitric acid 
with the precipitation of SD2O5. 

Stibnite may easily be distinguished from all minerals but the other 
sulphides by the test for sulphur. From the other sulphides it is dis- 
tinguished by its cleavage and the fumes it yields when heated on char- 
coal. Its closest resemblance is with galena (PbS), which, however, 
differs from it in being less fusible and in yielding a lead globule when 
fused with sodium carbonate on charcoal. Moreover, galena possesses 
a cubic cleavage. 

Syntheses. — Stibnite is produced by heating to 200 , a mixture of 
sulphur and antimony with water under pressure, and by the reaction of 
H2S on antimony oxide heated to redness. 

Occurrence, Localities and Origin.— -The mineral is found as crystals 
in quartz veins cutting crystalline rocks, and in metalliferous veins asso- 
ciated with lead and zinc ores, with cinnabar (HgS) and barite (BaS04). 
The finest crystals, some of them 20 inches in length, come from mines 
in the Province of Iyo, on the Island of Shikoku, Japan. The mineral 
occurs also in York Co., New Brunswick, in Rawdon township, Nova 
Scotia, at many points in the eastern United States, in Sevier Co., 
Arkansas, in Garfield Co., Utah, and at many of the mining districts in 
the Rocky Mountain States. 

In Arkansas stibnite is in quartz veins following the bedding planes 



74 DESCRIPTIVE MINERALOGY 

of shales and sandstones. With it are found many lead, zinc and 
iron compounds and small quantities of rarer substances. In Utah 
the mineral occurs in veins unmixed with other minerals, except its 
own oxidation products. The veins follow the bedding of sandstones 
and conglomerates. Here, as in Arkansas, the stibnite is believed to 
have been deposited by magmatic waters. 

Uses. — Stibnite was powdered by the ancients and used to color the 
eyebrows, eyelashes and hair. At present it is used to a slight extent in 
vulcanizing rubber and in the manufacture of safety matches, percussion 
caps, certain kinds of fireworks, etc. Its principal value is as an ore of 
antimony. Practically all of the metal used in the arts is obtained 
from this source. Antimony is chiefly valuable as an alloy with other 
metals. With tin and lead it forms type metal. The principal alloys 
with tin are britannia metal and pewter. With lead, tin and copper 
it constitutes babbit metal, a hard alloy used in the construction of 
locomotive and car journals, and with other substances it enters into 
the composition of other alloys used for a variety of purposes. The 
double tartrate of antimony and potassium is the well known tartar 
emetic. The pigment, Naples yellow, is an antimony chromate. 

Production. — The total quantity of stibnite mined in the world can- 
not be accurately estimated. That mined in the United States is very 
small in amount, most of the antimony produced in this country being 
obtained in the form of an antimony alloy as a by-product in the smelting 
of antimonial lead ores. 

Blsmuthinite (Bi 2 S 3 ) 

Bismuthinite is completely isomorphous with stibnite. It rarely, 
however, occurs in acicular crystals, but is more frequently in foliated, 
fibrous or dense masses. 

Its axial ratio is .968 : 1 : .985. 

The angle 1 10 A iTo = &&° 8'. 

The mineral resembles stibnite in color and streak, but its surface is 
often covered with a yellowish iridescent tarnish. Its fusibility and 
hardness are the same as those of stibnite but its density is 6.8-7.1. It 
is an electrical conductor. 

In the open tube the mineral yields SO2 and a white sublimate 
which melts into drops that are brown while hot, but change to opaque 
yellow when cold. On charcoal it yields a coating of yellow Bi203 which 
changes to a bright red BH3 when moistened with potassium iodide. 
The mineral dissolves in hot nitric acid, forming a solution, which upon 
the addition of water gives a white precipitate of a basic bismuth nitrate. 



SULPHIDES, TELLURIDES, ETC. 75 

Bismuthinite is distinguished from stibnite by the coating on char- 
coal and by its complete solubility in HNO3. 

Syntheses. — Crystals have been obtained by cooling a solution of 
Bi2Ss in molten bismuth, and by cooling a solu Jon made by heating 
IM2S3 in a solution of potassium sulphide in a closed tube at 200 . 

Occurrence, Localities and Origin. — Bismuthinite occurs as a constit- 
uent of veins associated with quartz, bismuth and chalcopyrite, in which 
it was probably formed as a product of pneumatolytic processes. It is 
found at Schneeberg and other points in Saxony; at Redruth and 
elsewhere in Cornwall; near Beaver City, Utah; in a gold-bearing vein 
at Gold Hill, Rowan County, N. C; and in a vein containing beryl, 
garnet, etc., in granite at Haddam, Conn. 

TETRADYMITE GROUP 

This group comprises a series of tellurides and selenides of bismuth 
that have not been satisfactorily differentiated because of the lack of 
accurate analyses. 

Tetradymite, the best known member of the group, is probably an 
isomorphous mixture cf bismuth telluride and bismuth sulphide of the 
formula Bi2(Te-S)3. It occurs in small rhombohedral crystals with the 
axial ratio 1 : 1.587 and 10T1 A 1101 = 98° 58'. Its crystals are bounded 
by rhombohedrons (R(ioTi) and 2R(202i)) and the basal plane 
(oP(oooi)). Interpenetration fourlings are common with — 5R(oil2), 
the twinning plane. The mineral is, however, more frequently found 
in foliated and granular masses. Its color is lead-gray. It possesses a 
perfect cleavage parallel to the base. Its hardness is 1.5-2 and its 
density about 7.4. It is a good electrical conductor. Its best known 
occurrences are Zsubkau, Hungary, Whitehall, Va., in Davidson 
County, N. C, near Dahlonega, Ga., near Highland, Mont., and at 
the Montgomery Mine and at Bradshaw City in Arizona. It occurs in 
quartz veins associated with gold in the gold sands of some streams. 

The other members of the group appear to be completely isomorphous 
with tetradymite. They vary in color from tin-white through gray to 
black. 



Molybdenite (MoS^ / 

This mineral, which is the sulphide of the rare metal molybdenum, 
does not occur in large quantity, but it is so widely distributed that it 
seems to be quite abundant. It occurs principally in black scales scat- 



76 DESCRIPTIVE MINERALOGY 

tered through coarse-grained, crystalline, siliceous rocks and granular 
limestones and in black or lead-gray foliated masses. 

The theoretical composition of molybdenite is 40 per cent sulphur 
and 60 per cent molybdenum. Usually, however, the mineral contains 
small quantities of iron and occasionally other components. 

Crystals of molybdenite are exceedingly rare. Scales and plates 
with hexagonal outlines are often met with but they do not usually pos- 
sess sufficiently perfect faces to yield accurate measurements. The 
measurements that have been obtained appear to indicate a holohedral 
hexagonal symmetry with an axial ratio 1 : 1.908. 

The cleavage of molybdenite is very perfect parallel to the base. 
The laminae are flexible but not elastic. The mineral is sectile and so 
soft that it leaves a black mark when drawn across paper. Its density 
is 4.7. Its luster is metallic, color lead-black, and streak greenish 
black. In very thin flakes the mineral is translucent with a green tinge. 
Otherwise it is opaque. It is a poor conductor of electricity at ordi- 
nary temperature, but its conductivity increases with the temperature. 

In the blowpipe flame molybdenite is infusible. It, however, im- 
parts to the edges of the flame a yellowish green color. Naturally, it 
fields all the reactions for sulphur, and in the open tube it deposits a 
pale yellow crystalline sublimate of M0O3. Molybdenite is decomposed 
by nitric acid with the production of a gray powder (M0O3). 

By its color, luster and softness molybdenite is easily distinguished 
from all minerals but graphite. From this it is distinguished by its 
reaction for sulphur. Moreover, a characteristic test for all molyb- 
denum compounds is the dark blue coating produced on porcelain when 
the pulverized substance is moistened with concentrated sulphuric 
acid and then heated until almost dry. Before this test can be applied 
to molybdenite, the mineral must first be powdered and then oxi- 
dized by roasting in the air for a few minutes or by boiling to dryness 
with a few drops of HNO3. 

Syntheses. — Crystalline molybdenite has been prepared by the action 
of sulphur vapor or H2S upon glowing molybdic acid. It has also been 
produced by heating a mixture of molybdates and lime, in a large excess 
of a gaseous mixture of HC1 and H2S. 

Occurrence, Localities and Origin. — Molybdenite generally occurs 
embedded as grains in limestone and in the crystalline silicate rocks, 
as, for instance, granite and gneiss, and as masses in quartz veins, at 
Arendal, Norway; at Blue Hill Bay, Maine; at Haddam, Conn.; in 
Renfrew Co., Ontario, and at many points in the far western states. 
It is thought to be of pneumatolytic origin. 



SULPHIDES, TELLURIDES, ETC. 77 

Uses. — The mineral is the principal ore of the metal molybdenum, 
the salts of which are important chemicals employed principally in 
analytical work, especially in the detection and estimation of phosphoric 
acid. The molybdate of ammonia (NH^MoOi, the principal salt 
employed in analytical processes, is easily obtained by roasting a mix- 
ture of sand and molybdenite and treating the oxidized product with 
ammonia. Other molybdenum salts are used for giving a green color 
to porcelain. The metal is used in an alloy (ferro-molybdenum) for 
hardening steel, as supports for the lower ends of tungsten filaments in 
electric lamps and for making ribbons used in electric furnaces. 

Production. — There was no production of molybdenite in North 
America during 191 2. The imports of the metal into the United States 
aggregated 3.5 tons, valued at $4,670. The value of the imports 
of the ore is not known, 

THE SULPHIDES, SELENIDES, ETC., OF THE METALS . 
THE METALLIC MONO SULPHIDES, ETC. 

The metallic monosulphides, monoselenides, etc., are compounds 
in which the hydrogen of H2S, H2Se, H2Te, H3AS, and IfcSb are 
replaced by metals. Among them are some of the most important 
ores. 

They may be separated into several groups of which some are 
among the best defined of all the mineral groups, while others consist 
simply of a number of minerals placed together solely for convenience 
of description. In addition, there are a few members of this chemical 
group which seem to have no close relationship with any other mem- 
bers. These are discussed separately. 

The groups described are as follows: 

The Dyskrasite Group. 
The Galena Group. 
The Chalcocite Group. 
The Blende Group. 
The Millerite Group. 
The Cinnabar Group. 

DYSKRASITE GROUP 

This group includes a number of arsenides and antimonides, some 
of which apparently contain an excess of the metal above that neces- 
sary to satisfy the formulas H3AS and HaSb. Although their com- 



78 DESCRIPTIVE MINERALOGY 

position is not understood, they are generally regarded as basic com- 
pounds. A few' of I hem are well crystallized, but their composition is 
dcubtful, because of the difficulty of obtaining pure material for anal- 
yses. Seme of them are probably mixtures. The members of the 
group, all of which are cemparatively rare, are whitneyiie (CugAs), 
algodonite (CueAs), domeykite (CU3AS), horsjordite (CuoSb) and dyskras- 
ite (Ag3Sb). Other minerals are known which may properly be placed 
here, but their identity is doubtful. The only two members that need 
further discussion are domeykite and dyskrasite. 

Domeykite (C113AS) is known only in disseminated particles and 
in botryoidal and dense masses and small orthorhombic crystals. It 
may be a mixture of several components, which in other proportions 
form algodonite. It is tin-white or steel-gray and opaque. It becomes 
dull and covered with a yellow or brown iridescent tarnish when ex- 
posed to the air. Its hardness is 3-4 and density about 7.3. It is the 
most easily fusible of the copper arsenides. Its principal occurrences 
are in the silver mines of Copiap6 and Coquimbo in Chile; associated 
with native copper at Cerro de Paracabas, Guerrero, Mexico; at Shel- 
don, Portage Lake, Michigan, and on Michipicoten Island, in Lake 
Superior, Ontario. The last two occurrences are in quartz veins. 

Dyskrasite (Ag3Sb) occurs in foliated, granular and structureless 
masses and rarely in small orthorhombic crystals with an hexagonal 
habit. Their axial ratio is .5775 : 1 : .6718. Twinning is frequent, 
yielding star-shaped aggregates. The mineral has a silver-white color 
and streak, but its exposed surfaces are often tarnished yellow or black. 
It is opaque and sectile. Its hardness is 3.5-4 and density about 9.6. 
It is a good electrical conductor. Dyskrasite is soluble in HNO3 
leaving a white sediment of SD2O3. It occurs principally in the silver 
mines of central Europe, and especially near Wolfach, Baden; St. 
Andreasberg, Harz; and at Carrizo, in Copiap6, Chile. 

GALENA GROUP 

The minerals comprising the galena group number about a dozen 
crystallizing in the holohedral division of the regular system (hex- 
octahedral class). They possess the general formula RQ in which 
R represents silver, lead, copper and gold, and Q sulphur, selenium 
and tellurium. The group may be divided into silver compounds and 
lead compounds, thus: (A) argentite (Ag2S), hessite (Ag2Te), petzite 
((Ag-Au)2Te), naumannite (Ag2Se), aguilarile (Ag2(Se-S)), jalpaite 



SULPHIDES, TELLURIDES, ETC. 79 

((Ag-Cu)2S) and eukarite ((Ag-Cu)2Se), and (B) galena (PbS), altaite 
(PbTe), and clausthalitc (PbSc). Of these only two are of importance, 
viz., galena, and argentite. Hessite and petzite are comparatively 
unimportant ores of gold. 

Argentite (Ag 2 S) 

Argentite, though not very widespread in its occurrence, is an 
important ore of silver. It is found in masses, as coatings, and in crys- 
tals or arborescent groups of crystals. 

Argentite contains 87.1 per cent silver and 12.9 per cent sulphur when 
pure. It is usually, however, impure through the admixture of small 
quantities of Fe, Pb, Cu, etc. 

The forms most frequently observed on argentite crystals are 
00 O 00 (100), ooO(no) and O(in), though various f»Ooo (hlo) and 
mOtn (Ml) forms are also met with. The crystals are often distorted 
and often they are grouped in parallel growths of different shapes. 
Twinning is common, with O(ni) the twinning plane. The twins 
are usually penetration twins. The habit of most crystals is cubical 
or octahedral. 

Argentite is lead-gray in color. Its streak is a little darker. The 
mineral is opaque. Its luster is metallic, its hardness about 2.25 and 
density 7.3. It is sectile, has an imperfect cleavage and is a conductor 
of electricity. 

When heated on charcoal argentite swells and fuses, yielding sulphur 
fumes and a globule of silver. It is soluble in nitric acid. 

Argentite is easily recognized by its color, its sectility, the fact that 
it yields a silver globule when fused with Na2CC>3 on charcoal and yields 
. the sulphur test with a silver coin. 

Syntheses. — Crystals of argentite may be obtained by treating red 
hot silver with sulphur vapor or dry H2S, and by heating silver and SO2 
in a closed tube at 200 . 

Occurrence, Localities and Origin. — The mineral is found in the second- 
ary enrichment zones of veins associated with silver and other sulphides 
in many silver-mining districts. In Nevada it is an important ore at 
the Comstock lode and in the Cortez district. It is found also near 
Port Arthur on the north shore of Lake Superior, in Ontario, and asso- 
ciated with native silver in the copper mines of Michigan. The ores of 
Mexico, Chile, Bolivia and Peru are composed largely of this mineral. 

Production. — Much of the silver produced in this country is obtained 
from argentite, though by no means so great a quantity as is obtained 
from other sources. 



Fe 


Zn 


SiOz 


Total 


i-35 


• • • 


■ • • 


99.96 


•36 


•IS 


.70 


100.01 


1.28 


.21 


.o<? 


100.00 



80 DESCRIPTIVE MINERALOGY 

Hessite (Ag 2 Te) and Petzite ((Ag*Au) 2 Te) 

These two minerals, though comparatively rare, are prominent 
sources of gold and silver in some mining camps. They usually occur 
together associated with other sulphides. 

Hessite is the nearly pure silver telluride and petzite, an isomorphous 
mixture of gold and silver tellurides, as indicated by the following analy- 
ses of materials from the Red Cloud Mine, Boulder Co., Colorado. 

Te Ag Au Cu Pb 

I. 37.86 59.91 .22 .17 .45 

II. 34.91 50.66 13.09 .07 .17 

III. 32.97 40.80 24.69 

The minerals crystallize in all respects like argentite. They are 
opaque and lead-gray to iron-black in color, sectile to brittle, have a 
hardness between 2 and 3 and a specific gravity of 8.3-9, increasing with 
the percentage of gold present. They are good conductors of electricity. 

Before the blowpipe, both minerals melt easily to a black globule, at 
the same time coloring the reducing flame greenish and giving the odor 
of tellurium fumes. When acted upon by the reducing flame, the globule 
becomes covered with little crystals of silver. With Na2C03 on charcoal 
both minerals yield a globule of silver, but the globule obtained from 
hessite dissolves in warm HNO3, while that obtained from petzite 
becomes yellow (gold). In the open tube both yield a white sublimate 
of TeCfe which melts, when heated, to colorless drops. When heated 
with concentrated H2SO4, they give a purple or red solution which, upon 
the addition of water, loses its color and precipitates blackish gray, 
powdery tellurium. The minerals dissolve in HNO3. From this solu- 
tion HCl throws down white silver chloride. 

Both the minerals resemble very closely many forms of argentite 
and galena, from which, however, they may be distinguished by the 
reactions for tellurium. Petzite and hessite may be distinguished from 
one another by the test for gold. Moreover a fresh surface of hessite 
blackens when treated with a solution of KCN, whereas a surface of 
petzite remains unaffected. 

Syntheses. — Octahedrons of hessite are obtained by the action of 
tellurium vapor upon glowing silver in an atmosphere of nitrogen, and 
dodecahedrons of petzite upon similar treatment of gold-silver alloy. 

Origin. — Both minerals are believed to be primary deposits orig- 
inating in magmatic solutions. They occur in veins with native gold, 
quartz, fluorite, dolomite, and various sulphides and other tellurides. 



SULPHIDES, TELLURIDES, ETC. 81 

Localities. — These tellurides, together with others to be described 
later (p. 113), are important sources of silver and gold in the mines at 
Nagyag, Transylvania, at Cripple Creek and in Boulder Co., Colo., and 
at Kalgoorlic, W. Australia. The quantity of tellurides mined is con- 
siderable, but since it is impracticable to separate these two tellurides 
from the other compounds of gold and silver mined with them, it is im- 
possible to estimate the proportion of the metals obtained from them. 

Galena (PbS) 

Galena, the most important ore of lead, occurs in great lead-gray 
crystalline masses, in large and small crystals, in coarse and fine granular 
aggregates, and in other less common forms. Much galena contains 
silver, in which case it becomes an important ore of this metal. 

Galena rarely approaches the theoretical composition 13.4 per cent 
cf sulphur and 86.6 per cent of lead. It usually contains small quanti- 
ties of the sulphides cf silver, zinc, cadmium, copper and bismuth and 
in some cases native silver and gold. When the percentage of silver 
present reaches 3 oz. per ton the mineral is ranked as a silver ore. This 
silver is apparently present in some cases as an isomorphous mixture 
of silver sulphide and in other cases in distinct 
minerals included within the galena. 

" Galena crystals usually possess a cubical habit, 
though crystals with the octahedral habit are 
very common. The principal forms observed are 
ooOoo(ioo), O(ni), ooO(no), mO<x>(hlo) and 
mOm (hll) (Figs. 29 and 30). Twins are common, 
with O the twinning face. FlG - ^.-Galena Crys- 

Galena is well characterized by its lead-gray , ' ~ ' /JX 

. (0); °°0, no (a) 

color, its perfect cleavage parallel to the cubic faces an d O, in (0). 
and by its great density (8.5). Its luster is me- 
tallic and its hardness about 2.6. Its streak is grayish black. It is a 
good conductor of electricity. 

On charcoal galena fuses, yielding sulphurous fumes and a globule 
of metallic lead, which may easily be distinguished from a silver globule 
by its softness. The charcoal around the assay is coated with a yellow 
sublimate of lead oxide (PbO). The mineral is soluble in HNO3 with 
the separation of sulphur. 

Its color and luster distinguish galena from nearly all minerals but 
5 Unite. From this mineral it is easily distinguished by its more difficult 
fusibility, by its cleavage, and by the fact that it does not yield the anti- 
mony fumes when heated on charcoal. 




82 DESCRIPTIVE MINERALOGY 

Galena weathers readily to the sulphate (angieske) and carbonate 
(cerussite); consequently it is usually not found in the upper portions 
of veins that are exposed to the action of the air. 

Syntheses.— Crystals of galena result from heating a mixture of 
lead oxide with NH4CI and sulphur, and from treatment of a lead salt 
with H2S at a red heat. Small crystals have been produced by heating 



Fir.. 30. — Galena Crystals («0™(ioo) and O(iti)) partly covered by Martasilc; 
Irom the Joplin Distri-.i, Mo. (Aft,r II'. S. T. Smith ,ind C. E. Submittal.) 

in a sealed glass tube at 8o°-oo° pulverized cerussite (PbCOa) in a water 
solution of H2S. 

Origin. — Veins of galena containing silver (silver-lead) were probably 
produced by ascending solutions emanating from bodies of igneous 
rocks, while the galena in limestone was probably deposited by ground- 
water that dissolved the sulphide from the surrounding sedimentary 
rocks. Galena is also in some cases a melamorphic product. 

Occurrence. — The mineral occurs very widely spread. It is found 
in veins associated with quartz (SiOs), calcite (CaCO-j), barite (BaSOi) 
or fluorite (CaF2) and various sulphides, especially the zinc sulphide, 
sphalerite; in irregular masses filling clefts and cavities in limestone; 



SULPHIDES, TELLURIDES, ETC. 83 

in beds, and in stalactites and other forms characteristic of water 
deposits. 

It occurs also as pseudomorphs after pyromorphite — the lead phos- 
phate. The form that occurs in veins is often silver bearing, while that 
in limestone is usually free from silver. 

Localities. — Galena is mined in Cornwall and in Derbyshire, Eng- 
land; in the Moresnet district, Belgium; at various places in Silesia, 
Bohemia, Spain and Australia. In the United States it occurs in veins at 
Lubec, Me., at Rossie, St. Lawrence Co., N. Y., at Phoenixville, Penn., at 
Austin's Mines in Wythe Co., Va., and at many other places. It is 
mined for silver in Mexico; at Leadville, Colo.; at various points in 
Montana; in the Cceur d'Alene region in Idaho and at many other places 
in the Rocky Mountain region. 

The most extensive galena deposits in this country are in Missouri; 
in the corner made by the states of Wisconsin, Illinois and Iowa; and 
in Cherokee Co., Kansas. In these districts the galena, associated 
with sphalerite (ZnS), pyrite (FeS2), smithsonite (ZnCOa), calamine 
((ZnOH)2Si03), cerussite (PbCOa), calcite (CaCCte) and other minerals, 
fills cavities in limestone. 

Extraction of Lead and Silver from Galena. — The ore is first crushed 
and concentrated by mechanical or electrostatic methods, and the 
concentrates are roasted to convert them into oxides and sulphates. 
The mass is then heated without access of air, sulphur dioxide being 
driven off, leaving metallic lead carrying impurities, cr a mixture of 
lead and silver. 

The processes employed in refining the impure lead vary with the 
nature of the impurities. 

Uses. — Galena is employed to some extent in glazing common 
stoneware. It is also used in the preparation of white lead and other 
pigments. As has alrcr.c!y been stated, it is the most important ore of 
lead and a very important ore of silver. 

The metal lead finds many uses in the arts. Its most common 
use is for piping. Its alloys, type metal, pewter and babbitt metal 
have already been referred to (p. 74). Solder is an alloy of tin and lead; 
Wood's metal a mixture of lead, bismuth, tin and cadmium. The spe- 
cial characteristic of Wood's alloy is its low fusion point (70 ). 

Production. — The total production of galena by the different coun- 
tries of the world cannot be given, but the world's production of lead 
in 191 2 was 1,277,002 short tons. The total quantity of lead pro- 
duced by the United States from domestic ores in the same year was 
about 4i5>395 tens, valued zt $37,385,550. Most of this was obtained 



g4 DESCRIPTIVE MINERALOGY 

from galena. About 171,037 tons were soft lead, smelted from ores 
mined mainly for their lead and zinc contents, and the balance from 
ores mined partly for their silver. The importance of galena as an ore 
of silver may be appreciated from the fact that of the $39,197,000 
worth of this metal produced in the United States during 191 2, silver 
to the value of about $12,000,000 was obtained from lead ores or from 
mixtures of lead and zinc ores. 

Altaite (PbTe) and clausthalite (PbSe) both resemble galena in 
appearance. Both occur commonly in fine-grained masses, but they 
are also found in cubic crystals. Altaite is tin-white, tarnishing to 
yellow or bronze, and clausthalite is lead-gijay. Their hardness is 2.5-3 
and specific gravity about 8.1. They are associated with silver and lead 
compounds principally in the silver mines of Europe and South America. 
Altaite is known also from several mines in California, Colorado and 
North Carolina. They are distinguished from one another and from 
galena by the tests for Te and Se. 

CHALCOCITE GROUP 

The chalcocite group includes four or five cuprous and argentous 
sulphides, selenides and tellurides. They all crystallize in the ortho- 
rhombic system (rhombic bipyramidal class) often with an hexagonal 
habit, and are isomorphous. The best known members of the group 
are chalcocite (CU2S) and stromeyerite (Cu-Ag)2S, but only the first- 
named is common. Although these minerals are orthorhombic, never- 
theless CU2S is known to exist also in isometric crystals, in which form 
it is isomorphous with argentite. Moreover, stromeyerite is an iso- 
morphous mixture of Ag2S and CU2S. Therefore, it is inferred that 
CU2S and Ag2S are isomorphous dimorphs. 

Chalcocite (Cu 2 S) 

Chalcocite (CU2S), the cuprous sulphide, is an important ore of 
copper though by no means as widely spread as the iron-copper sul- 
phide, chalcopyrite. It is usually found in black masses with a dull 
metallic luster and as a black powder, though frequently also in crys- 
tals. It is a common constituent of the enrichment zone of many veins 
of copper ores. 

The best analyses of chalcocite agree closely with the formula 
given above, requiring the presence of 20.2 per cent of sulphur and 
79.8 per cent of copper. Iron and silver are often present in the mineral 
in small quantity. 



SULPHIDES, TELLURIDES, ETC. 85 

In crystallization chalcocite is orthorhombic (rhombic bipyramidal 
class) with the axial ratio .5822 : 1 : .9701. Its crystals contain as 
their predominant forms oP(ooi), ooP(no), «Po&(oio), P(ni), 

a series of prisms cf the general symbol — P(nA), and several bra- 

chydomes. Many cf the crystals are elongated parallel to H, and 
others are so developed as to possess an hexagonal habit (Fig. 31). 
Twins are common according to several laws. When the twinning plane is 
JP (n a) the twins are usually cruciform (Fig. 31). The zone 001— 010 
is often striated through oscillatory combinations. iioaiio=6o° 25'. 
The cleavage of chalcocite is indistinct, its fracture is conchoidal. 
Its hardness is 2.5-3 an ^ density about 5.7. Its streak, like its color, 




Fie. 31. Fig. 3a. 

Fie. 31.— Chalcocite Crystal. oP, 001 (c); °oP«, 010(b); »P, no (m); iPw , 

021 id); JP«,02 3 («); P. i" (?)andiP, 113 «■ 

Fio. 31.— Complex Chalcocite Twin, with »P, no (m) and )P, 112 {») the Twinning 

Planes. 

is nearly black, but exposed surfaces are often tarnished blue or green, 
probably through the production of thin films of other sulphides like 
covelUte (CuS), chalcopyrite (FeCuSa),etc. The mineral is an excel- 
lent conductor of electricity. 

In the open tube or on charcoal chalcocite melts and yields sul- 
phurous fumes. 

When mixed with NazCOs and heated a copper globule is produced. 
The mineral dissolves in nitric acid with the production of a solution 
that yields the test for copper. 

Upon exposure to the air chalcocite changes readily to the oxide, 
cuprite (CU2O), and the carbonates, malachite and azurite. In the 
presence of silicious solutions it may give rise to the silicate, chrysocolla 
(p. 441). 

A pseudomorph of chalcocite after galena is known as harrisiU. 



86 DESCRIPTIVE MINERALOGY 

It occurs at the Canton Mine in Georgia and in the Polk Co. copper 
mines in Tennessee. Pseudomorphs after many other copper min- 
erals are common. 

Chalcocite is recognized by its color and crystallization. Massive 
varieties are distinguished from or gentile by greater brittleness and the 
reaction for copper; from bornite (CuaFeSa) by the fact that it is not 
magnetic after roasting. 

Syntheses. — Crystals of chalcocite have been made in many ways, 
more particularly by heating the vapors of CuCk and H2S, and by 
gently warming CU2O in H2S. Measurable crystals have been observed 
on old bronze that has been immersed in the waters of hot springs for 
a long time. 

Occurrence, Localities and Origin. — The mineral is a common prod- 
uct of the alteration of other copper compounds in the zone of secondary 
enrichment of sulphide veins. It is therefore present at most localities 
of copper minerals. One of the best known occurrences is Butte, 
Mont. 

Fine crystals of chalcocite occur in veins and beds at Redruth and 
at other places in Cornwall, England; at Bristol in Connecticut, and 
at Joachimthal in Bohemia. The massive variety is known at many 
places. In the United States it occurs in red sandstone at Cheshire 
in Connecticut. It is found also in large quantities near Butte City in 
Montana, and in Washoe and other counties in Nevada, and indeed 
in the veins of most copper producing mines. In Canada it is present 
with chalcopyrite and bornite at Acton, Quebec, and at several places 
in Ontario north of Lake Superior. 

Extraction of Copper. — Chalcocite rarely occurs alone in large 
quantity. In ores it is usually mixed with other compounds of copper, 
and is treated with them in extracting the metal (see p. 133). 

Stromeyerite ((Ag-Cu)2S) is usually massive, but it occurs also in 
simple and twinned crystals similar to those of chalcocite. Their axial 
ratio is .5822 : 1 : .9668, almost identical with that of chalcocite. The 
mineral is opaque and metallic. Its color and streak are dark steel- 
gray. Its hardness is 2.5-3 an d density about 6.2. It is soluble in 
nitric acid. It occurs associated with other sulphides in the ores of 
silver and copper mines at Schlangenberg, Altai; Kupferberg, Silesia; 
Coquimbo, Copiapo, and other places in Chile, and in a few mines in 
California, Arizona, and Colorado. 



SULPHIDES, TELLURIDES, ETC. 87 



BLENDE GROUP 

The blende group of minerals comprises a series of compounds whose 
general formula like that of the galena group is RQ. In the blendes R 
stands for Zn, Cd, Mn, Ni and Fe and Q for S, Se and Te. 

The blendes are all transparent or translucent minerals of a lighter 
color than galena. They constitute an isodimorphous group of a dozen 
or more members crystallizing in the tetrahedral division of the regular 
system (hextetrahedral class), and in hemimorphic holohedral forms of 
the hexagonal system (dihexagonal-pyramidal class). The group may 
be divided into two subgroups known respectively as the sphalerite 
and the wurtzite groups. 

SPHALERITE DIVISION 

The most important member of this division of the blende group is 
the mineral sphalerite. This, like the other less well known members, 
crystallizes in the hemihedral division of the regular system with various 
tetrahedrons as prominent forms. The other members of the group 
are alabandite (MnS), and an isomorphous mixture of FeS and NiS, 
pentlandite. 

Sphalerite (ZnS) 

Sphalerite, one of the very important zinc ores and one of the most 
interesting minerals from a crystallographic standpoint, occurs in amor- 
phous and crystalline masses and in handsome crystals and crystal groups. 
Botryoidal and other imitative masses are common. 

Pure white sphalerite consists of 67 per cent of Zn and 23 per cent of 
sulphur. The colored varieties usually contain traces of silver, iron, 
cadmium, manganese and other metals. Sometimes the proportion of 
the impurities is so large that the mineral containing them is regarded as 
a distinct variety. Two analyses of American sphalerites are as follows: 

S Zn Cd Fe Total 

Franklin Furnace, N. J 32 . 22 67 .46 tr ... 99.68 

Joplin, Mo 3 2 -93 66.69 ••• -4 2 100.04 

The hemihedral condition of sphalerite is shown in the predominance 
of tetrahedrons among its crystal forms and by the symmetry of its 

etched figures (Fig. 33). Its most common forms are — - — (321) and 

2 

2O lO 

other hextetrahedrons, d= — (221), — (331) and other deltoid-dodeca- 

2 2 



88 DESCRIPTIVE MINERALOGY 

hedrons and ±303(311) and other tris tetrahedrons. In addition, 
00 O 00 (100) and 00 0(i 10) are quite common (Fig. 34). Twins are 
abundant. Their twinning plane is O and their composition face either 
(Fig. 35), or a plane perpendicular to this. Through twinning, the 
crystals often assume a rhombohedral habit. 

The cleavage of sphalerite is perfect parallel to 00 O(no). From a 
compact mass of the mineral a fairly good dodecahedron may some- 
times be split. Its fracture is conchoidal. When pure the mineral is 
transparent and colorless. As usually found, however, it is yellow, 
translucent and black, brown, or some shade of red. Its streak is 
brownish, yellow or white. The yellow masses look very much like 



FlO. 33.— Telrahedral Crystal of Sphalerite Bounded by »0»(ioi) and ±0 (11 1 
and ill), Illustrating the Fact that Its Octahedral Faces Fall into Two Groups. 

Fie. 34.— Sphalerite Crystal: «0, no (rf), and+— ^, 311 (m). 

Fig. 35.— Sphalerite Octahedron Twinned about 0(m). 

lumps of rosin. The hardness of sphalerite is between 3.5 and 4, and its 
density about 4. Its luster is resinous. The mineral is difficultly fusible, 
and is a nonconductor of electricity. Its index of refraction («) for 
yellow light is 2.369. 

Sphalerite when powdered always yields tests for sulphur under 
proper treatment. On charcoal it volatilizes slowly, coating the coal 
with a yellow sublimate when hot, turning white on cooling. When 
moistened with a dilute solution of cobalt nitrate and heated in the 
reducing flame, the white coating of ZnO turns green. The mineral dis- 
solves in hydrochloric acid, yielding sulphuretted hydrogen. 

By oxidation sphalerite changes into the sulphate of zinc, and by 
other processes into the silicate of zinc, calamine, or the carbonates, 
smithsonite and hydrozincite. 



SULPHIDES, TELLURIDES, ETC. 89 

Syntheses. — Sphalerite crystals have been made by the action of H2S 
upon zinc chloride vapor at a high temperature. They are also often 
produced in the flues of furnaces in which ores containing zinc and sul- 
phur are roasted. 

Occurrence and Origin. — Sphalerite occurs disseminated through lime- 
stone, in streaks and irregular masses in the same rock, and in veins cut- 
ting crystalline and sedimentary rocks. It is often associated with 
galena. The material in the veins is often crystallized. Here it is asso- 
ciated with chalcopyrite (CuFeS2), fluorite (CaF2), barite (BaSCU), 
siderite (FeCOa), and silver ores. When in veins it is in some cases the 
result of ascending hot waters and in other cases the product of down- 
ward percolating meteoric water. Much of the disseminated ore is a 
metamorphic contact deposit. 

Localities. — Crystallized sphalerite is found abundantly at Alston 
Moor, Cumberland, England; at various places in Saxony; in the Bin- 
nenthal, Switzerland; at Broken Hill, N. S. Wales, and in nearly all 
localities for galena. Handsome, transparent, cleavable masses are 
brought from Pilos de Europa, Santander, Spain. Stalactites are 
abundant near Galena, 111. 

The principal deposits of economic importance in America are those 
in Iowa, Wisconsin, Missouri and Kansas, where the sphalerite is asso- 
ciated with other zinc compounds and with galena forming lodes in 
limestone, and at the silver and gold mines of Colorado, Idaho and Mon- 
tana, 

Extraction of the Metal. — In order to obtain the metal from sphalerite, 
the ore is usually first concentrated by flotation or other mechanical 
processes. The concentrates are then converted into the oxide by roast- 
ing and the impure oxide is mixed with fine coal and placed in clay retorts 
opening into a condenser. These are gradually heated. The oxide is 
reduced to the metal, which being volatile distils over into the con- 
denser, where it is safely caught. Other processes are based on wet 
chemical methods. 

Uses of Zinc. — Zinc is used extensively in galvanizing iron wire and 
sheets. It is also employed in the manufacture of important alloys 
such as brass, and in the manufacture of zinc white, which is the oxide 
(ZnO), and other pigments. A solution of the chloride is used for pre- 
serving timber. Argentiferous zinc is the source of a considerable quan- 
tity of silver. 

Production. — The figures showing the quantity of sphalerite pro- 
duced in the zinc-producing countries are not available. The total 
amount of metallic zinc produced in the year 191 2 was 1,070,045 tons, 



90 DESCRIPTIVE MINERALOGY 

valued at $44,699,166, of which the United States produced from domestic 
ores 323,907 tons, and in addition used, in the making of zinc compounds, 
about 55,000 tons. Of this aggregate, Missouri produced about 149,560 
tons. Most of the metal was obtained from sphalerite, but a large 
part came from other ores. The quantity of silver produced in refining 
zinc ores was 664,421 oz., valued at $408,619. 

Alabandite (MnS) is isomorphous with sphalerite. It usually 
occurs, however, in dense granular aggregates of an iron-gray color. 
Its streak is dark green. It is opaque and brittle. Its hardness is 3-4 
and density 3.9. It is not an electrical conductor. When heated on 
charcoal in the reducing flame it changes to the brown oxide of man- 
ganese and finally melts to a brown slag. It is soluble in dilute HC1 
with the evolution of H2S. Alabandite occurs with other sulphides at 
Kapnik, Hungary; at Tarma, Peru; at Puebla, Mexico, and in the 
United States at Tombstone, Arizona, and on Snake River, Summit Co., 
Colorado. 

Pentlandite ((Fe • Ni)S) may belong to this group. Iron is frequently 
found in crystallized sphalerite. Its sulphide, therefore, may be isomor- 
phous with sphalerite, in which case pentlandite, which is probably an 
isomorphous mixture of NiS and FeS, would also belong in the sphal- 
erite group. The mineral occurs in light bronzy yellow, granular masses 
with a distinct octahedral cleavage, a hardness of 3.5-5 and a density of 
4.6. It is a nonconductor of electricity. Pentlandite occurs with 
chalcopyrite (CuFeS2) and pyrrhotite (FeySg), at Sudbury, Ontario, 
where it is probably the constituent that furnishes most of the nickel 
(see p. 92). 

It is distinguished from pyrrhotite, which it resembles in appearance, 
by its cleavage and the fact that it is not magnetic. Moreover, it 
weathers to a brassy yellow color, while pyrrhotite weathers bronze. 

WURTZITE DIVISION 

The wurtzite group comprises only two or three members, wurtziie 
(ZnS), greenockite (CdS), and possibly pyrrhotite (Fe n S n+1 ). All crys- 
tallize in the holohedral division of the hexagonal system and the first 
two are unquestionably hemimorphic (dihexagonal pyramidal class). 
Pyrrhotite is the most common. 

Wurtzite (ZnS) is one of the dimorphs of ZnS, sphalerite being the 
other. It occurs in brownish black crystals, in masses and in fibers. 



SULPHIDES, TELLURIDES, ETC. 91 

Its crystals are combinations of ooP(ioTo) with 2P(202i) and 
oP(oooi) at one end, and a series of steeper pyramids at the 
other. Their axial ratio is i : .8175. The angle 10T1 A 01^1 = 40° 9'; 

2P(022l) A2P(022l) = 52° 27'. 

The mineral is brownish black to brownish yellow and its streak 
is brown. Its hardness is between 3 and 4 and its sp. gr. is about 4. 
It conducts electricity very poorly. In chemical and physical prop- 
erties it resembles sphalerite. Its crystals have been produced by 
fusing a mixture of ZnS04, fluorite and barium sulphide. They are 
frequently observed as furnace products. 

Wurtzite occurs as crystals at the original Butte Mine, Butte, 
Montana, and in a mine near Benzberg, Rhenish Prussia, at both 
places associated with sphalerite. They also occur with silver ores near 
Oruro and Chocaya, Bolivia, and near Quispisiza, Peru. 

Greenockite. — Greenockite (CdS) is completely isomorphous with 
wurtzite. Its crystals have an axial ratio 
1 : .8109. In general habit they are like 
those of wurtzite but they contain many more 
planes (Fig. 36). The angle io7iaoi^i = 
39 58'. Crystals are rare and small. The 
mineral usually occurs as a coating on other 
minerals, especially sphalerite. Its color is 
honey to orange-yellow, its streak orange- Fig. 36.— Greenockite Crys- 
yellow, and its luster glassy or resinous. It taL 00 p, ioTo^(m); 2P, 
is transparent or translucent and is brittle. 2021 («); P, ion (*), and 

Its hardness is 3-3.5 and density about 4.0. ? ' -/f . Y. e orm 

° J . J "* * JP, 1012 (i) is often pres- 

Its index of refraction = 2.688. When C nt at the upper end of 

heated in the closed tube it becomes carmine, the crystals.) 

but it changes to its original color on cooling. 

It yields the usual reactions for sulphur and cadmium, and dissolves 

in HC1, yielding H2S. 

Crystals have been obtained by melting a mixture of CdO, BaS, 
and CaF2, and by heating cadmium in an atmosphere of H2S to near 
fusing point. The mineral is a common furnace product. Greenockite 
crystals occur with prehnite at Bishoptown, Scotland, and as coatings 
on sphalerite in the zinc regions of Missouri and Arkansas, and at 
Friedensville, Pennsylvania. 




92 DESCRIPTIVE MINERALOGY 



Pyrrhotite (Fe n Sn+i) 

Pyrrhotite, or magnetic pyrite, occupies the anomalous position 
of being one of the most important ores of nickel, whereas it is essen- 
tially a sulphide of iron. The name is really applied to a series of 
compounds whose composition ranges between FesSe and FeieSiy. 
The crystallized material is in some cases FerSg, and in others, FenSi2. 
It is probably a solid solution of FeS2 or S in the sulphide of iron (FeS). 
As usually found, pyrrhotite is in bronze-gray granular masses, that 
tarnish rapidly to bronze on exposure to the air. Good crystals of 
the mineral are rare. 

Analyses of pyrrhotite vary widely. The percentages of Fe and S 
corresponding to FeySg are Fe, 60.4; S, 39.6, and those corresponding 
to FenSi2 are Fe, 61.6; S, 38.4. Much of the mineral contains in addi- 
tion to the iron and sulphur sufficient nickel to render it an ore of this 
metal, but it is probable that the nickel is present in pentlandite (see 
p. 90) or some other nickel compound embedded in the pyrrhotite. 

Analyses of pyrrhotite from various localities are: 

S Fe Co Ni Total 

Schneeberg, Saxony 39 . 10 61.77 tr IO ° • 87 

Brewster, N. Y 37 . 98 61 . 84 ... .25 100.07 

Sudbury, Ontario 38. 91 56.39 .... 4.66 99 .96 

Gap Mine, Penn 38. 59 55 .82 5.59 100.00 

The few crystals of pyrrhotite known are distinctly hexagonal in 

habit with a : c=i : 1.7402. They are com- 
monly tabular or acutely pyramidal, but it 
has not been established that they are hemi- 
m —* morphic, although the almost universal pres- 
ence of FeS in crystals of wurtzite would 
Fig. 37.-Pyrrhotite Crystal. ^^^ that the two substances are isomor- 
oP, 0001 (c); P, 1011 (5); , „,, . , , . . , , 

P 4041 (u) and 00 p Ph° us ' The tabular crystals possess a broad 

10T0 (m). ' basal plane, which surmounts hexagonal prisms 

ooP(ioTo) and ooP2(ii2o), and a series of 
pyramids, of which 2P(202i), iP(ioi2), P(ioTi) and P2(ii22) are the 
most frequent. (Fig. 37.) The angle 10T1 AoiTi = 53° n'. 

The cleavage of pyrrhotite is not always equally distinct. When 
marked it is parallel to 00 P2 (11 20). There is also often a parting 
parallel to the base. Its fracture is uneven. The mineral is brittle. 
It is opaque, and has a metallic luster. Its color varies between bronze- 








SULPHIDES, TELLURIDES, ETC. 93 

yellow and copper-red, and its streak is grayish black. Its hardness is 
a little less than 4 and its density about 4.5. All specimens are magnetic 
but the magnetism varies greatly in intensity, being at a maximum in 
the direction of the vertical axis. The mineral is a good conductor of 
electricity. 

Pyrrhotite gives the usual reactions for iron and sulphur, and some- 
times, in addition, the reactions for cobalt and nickel. It is decom- 
posed by hydrochloric acid with the evolution of H2S, which may 
easily be detected by its odor. 

From the many sulphides more or less closely resembling pyrrhotite 
in appearance, this mineral may easily be distinguished by its color 
and density and by its magnetism. 

Syntheses. — Crystals may be obtained by heating iron wire or 
Fea04, or dry FeCb to redness in an atmosphere of dry H2S and by 
heating Fe in a closed tube with a solution cf f eCfe saturated with 
H 2 S. 

Occurrence, Localities and Origin. — Pyrrhotite occurs completely 
filling vein fissures, and also as crystals embedded in other minerals 
constituting veins. It occurs also as impregnations in various rocks 
and as a segregation in the coarse-grained basic rock known as norite, 
where it is believed to have separated from the magma producing the 
rock. It may also in some cases be a product of metamorphism on the 
borders of igneous intrusions. 

It is found at Andreasberg, Harz; Bodenmais, Bavaria; Minas 
Geraes, Brazil; various points in Norway and Sweden, and on the 
lavas of Vesuvius. In North America crystals occur at Standish, Maine; 
at Trumbull, Monroe Co., N. Y.; and at Elizabethtown, Ontario. 
The mineral has been mined at Ducktown, Tenn.; at Ely, Vermont, 
and at Gap Mine, Lancaster Co., Penn. 

Its mines at present, however, are at Sudbury, in Ontario, where the 
mineral is associated with magnetite, chalcopyrite and pentlandite 
((Fe-Ni)S) on the lower border of a great mass of igneous rock (norite). 
Besides these there are present also embedded in the pyrrhotite 
small quantities of other minerals, so that the ore as mined is very 
complex. 

Pyrrhotite is sometimes found altered to pyrite, to limonite and to 
siderite (FeCOa). 

Extraction. — Pyrrhotite is crushed and roasted to drive off the 
greater portion of the sulphur. It is then placed in a furnace and 
smelted with coke and quartz. The nickel, copper and some of the 
iron, together with some of the fused sulphides, collect as a matte in the 



94 DESCRIPTIVE MINERALOGY 

bottom of the furnace from which it is withdrawn from time to time. 
The matte is riext roasted to transform the iron it contains into oxides 
and the remaining nickel and copper are separated by patented or secret 
methods. 

Uses. — The mineral is sometimes worked for the sulphur it con- 
tains. Its principal use, however, is as a source for nickel, nearly all of 
this metal used in America coming from the nickeliferous variety found 
at Sudbury, Ontario. 

The metal nickel has come into extensive use in the past few years 
in connection with the manufacture of armor plate for warships. The 
addition of a few per cent of nickel to steel hardens it and increases 
its strength and elasticity. 

Nickel is also extensively used in nickel-plating and in the manufac- 
ture of alloys. German silver is an alloy of nickel, copper and zinc. The 
nickel currency of the United States contains about 25 per cent Ni and 
75 per cent Cu. Monel metal is a silver- white alloy containing about 
75 per cent Ni, 1 per cent Fe and 29 per cent Cu. It is stronger than 
ordinary steel, takes a brilliant finish and is impervious to acids. It is 
made directly at Sudbury, Ont., by smelting. 

Production. — The production of pyrrhotite and chalcopyrite (CuFeS) 
at the Sudbury mines in 191 2 amounted to 737,584 short tons. The 
value of the matte produced was $6,303,102, and the value of nickel con- 
tained in it was about $16,000,000. About half of the nickel was used 
in America; the remainder, amounting to $8,515,000, was exported, after 
being refined in the United States. Formerly the United States pro- 
duced a considerable quantity of nickel from domestic ores, most of 
it from pyrrhotite, but the mines have been closed down within the past 
few years. It is, however, produced as a by-product in the refining 
of copper ores to the amount of about 325 tons annually, This is worth 
about $260,000 (see also p. 400). 

MILLERITE GROUP 

This group comprises sulphides, arsenides and antimonides of nickel. 
It includes the minerals mUlerite (NiS) 9 niccolite (NiAs), arite (Ni(Sb- As)) 
breithauptite (NiSb) and a few others. Of these only milled te and nic- 
colite are at all common. The minerals all crystallize in the hexagonal 
system, possibly in the rhombohedral division (ditrigonal scalenohedral 
class). Well defined crystals are, however, rare and often capillary so 
that their symmetry has not been determined with certainty. 



SULPHIDES, TELLURIDES, ETC. 95 



Millerite (NiS) 

Millerite is easily recognized by its brass-yellow color. It occurs 
most frequently in slender hair-like needles, often aggregated into tufts 
or radial groups, or, woven together like wads of hair, forming coatings 
on other minerals. 

Pure millerite contains 35.3 per cent sulphur and 64.6 per cent nickel. 
It frequently contains also a little Co and Fe. 

Crystals are thin, acicular or columnar with prismatic and rhom- 
bohedral faces predominating, and an axial ratio of 1 : .330, or of 1 : .9886 
if the rhombohedron 311(0331) is taken as the ground form. 

The mineral is elastic. Its hardness is 3-3.5 and density about 5.5. 
It is opaque and brassy yellow. Its streak is greenish black. It is an 
excellent conductor of electricity. 

The mineral yields sulphurous fumes in the open tube. After roast- 
ing it gives, with borax and microcosmic salt, a violet bead when heated 
in the oxidizing flame of the blowpipe. On charcoal with Na2C03 it 
yields a magnetic globule. 

Synthesis. — Bunches of yellow acicular crystals of NiS have been 
formed by treatment of a solution of NiSO* with H2S, under pressure. 

Localities. — Millerite occurs as long acicular crystals in cavities in 
other minerals at Joachim thai, in Bohemia, and at many places in 
Saxony. In the United States it forms radiating groups in cavities in 
hematite (Fe20s) at Antwerp, N. Y. At the Gap Mine, Lancaster Co., 
Penn., it forms coatings on other minerals and at St. Louis, Mo. and 
at Milwaukee, Wis., it occurs in delicate tangled tufts in geodes in lime- 
stone. Nowhere does it occur in sufficient quantity to constitute an ore* 

Niccolite (NiAs) 

Niccolite usually occurs massive, though crystals are known. It is 
of economic importance only in a few localities. 

Theoretically, the mineral contains 56.10 per cent As and 43.90 per 
cent Ni, but as usually found it contains also Sb, S, Fe and often small 
quantities of Co, Cu, Pb and Bi. 

Its crystals, which are rare, are hexagonal and hemimorphic (prob- 
ably dihexagonal pyramidal class), with 0:1=1: .8194. The prism 
ooP(ioTo), and oP(oooi) are the predominant forms, with the 
pyramids P(ioTi) and ^(5057) less well developed. The angle 

io7i aoi^i == 4o° i 2 '- 

The mineral is pale copper-red and opaque. It has a brownish 



fl» DESCRIPTIVE MINERALOGY 

black streak. Its hardness is about 5 and its density 7.6. The surfaces 
of nearly all specimens are tarnished with a grayish coating. The min- 
eral is a good conductor of electricity. 

In the open tube niccolite yields arsenic fumes and often traces of 
SO2. On charcoal with Na2COa it yields a metallic globule of nickel. 
It dissolves in HNO3 with the precipitation of AS2O3. The apple-green 
solution, thus produced, becomes sapphire-blue on addition of ammonia. 

Its peculiar light pink color and its reactions for arsenic and nickel 
distinguish niccolite from all other minerals, except, perhaps, breit- 
hauptitCy which, however, contains antimony. 

Occurrence. — Niccolite occurs principally in veins in crystalline 
schists and in metamorphosed sedimentary rocks, associated with silver 
and cobalt sulphides and arsenides. 

Localities. — The principal locality for niccolite in North America is 
Cobalt, Ontario, where it is found with native silver and silver, cobalt, 
and other nickel compounds, all of which are thought to have been de- 
posited by hot waters emanating from a mass of diabase. In Europe it 
is abundant at Joachimsthal in Bohemia, and at a number of other 
places in small quantity. 

Although rich in nickel, the mineral is not used as an ore at present, 
except to a very minor extent, most of the nickel of commerce being 
obtained from other compounds (see p. 94). 

Breithauptite (NiSb) is rare. It is of a light copper-red color, much 
brighter than that of niccolite, and its streak is reddish brown. Its hard- 
ness is 5.5 and density about 7.9. Its crystals are hexagonal tables 
with an axial ratio 1 : 1.294, and a distinct cleavage parallel to oP(ooi). 
It usually occurs in dendritic groups, in foliated and finely granular 
aggregates and in dense masses. It is a frequent furnace product, when 
ores containing Ni and Sb are smelted. It is found at Andreasberg, Harz ; 
at Sarrabus, in Sardinia; at Cobalt, Ont., and at a few other places. It 
is distinguished from niccolite by its deeper color and its content of Sb. 



Covellite (CuS) 



Covellite, or indigo copper, is the cupric sulphide, chalcocite being 
the corresponding cuprous salt. It is called indigo copper because of 
the deep blue color of its fresh fracture. It is often mixed with other 
copper compounds from which it has been derived by alteration. It 
usually occurs massive, but crystals are known. It is an unimportant 
ore of copper. 



SULPHIDES, TELLURIDES, ETC. 97 

The theoretical composition of the mineral is 33.56 per cent S, 
66.44 P er cent Cu. It usually, however, contains also a little iron and 
often traces of lead and silver. 

Crystals of. covellite are not common. They are hexagonal with 
a : c= 1 : 3.972 and their habit is usually tabular. The forms observed 
are oP(oooi), 00 P(ioTo), P(ioTi) and JP(iol4). 10T1 Aoi^i = 77° 42'. 

The mineral has one perfect cleavage parallel to oP(oooi). In 
thin splinters it is flexible. Its hardness is 1.5-2 and density about 
4.6. Its color is dark blue and its streak lead-gray to black. It is 
opaque, with a luster that is sometimes nearly metallic, but more 
frequently dull. It is a good electrical conductor. 

The blowpipe reactions of covellite are like those of chalcocite, with 
these exceptions: Covellite burns with a blue flame when heated on 
charcoal, and yields a sublimate of sulphur in the closed tube. 

Covellite is distinguished from other minerals than chalcocite by 
its reactions for Cu and S and the absence of reactions for Fe. It is 
distinguished from chalcocite by its color and density and by the fact 
that it ignites on charcoal. 

Syntheses. — The treatment of green copper carbonate with water 
and H2S in a closed tube at 8o°-9o° yields small grains of covellite. 
The mineral has also been produced by the action of H2S upon vapor 
of Q1Q2, and by treating sphalerite with a solution of copper sulphate 
in a sealed glass tube containing CO2 at a temperature of i5o°-i6o° 
for two days. 

Localities and Origin. — The mineral is comparatively rare. It is 
abundant in Chile and Bolivia and at Butte, Mont., and is found in 
crystals on the lava of Vesuvius and elsewhere. It usually occurs as 
an alteration product of other copper-sulphur compounds, especially in 
the zone of secondary enrichment of copper veins. 

Uses. — It is mined with other compounds and used as a source 
of copper. 

CINNABAR GROUP 

This group comprises sulphides, selenides and tellurides of mercury. 
The group is dimorphous, with its members crystallizing in hemihedrons 
of the isometric system (hextetrahedrai class) and in tetartohedrons 
of the hexagonal system (trigonal trapezohedral class). The isometric 
HgS is known as metacinndbarite and the hexagonal form as cinnabar. 
Only the latter is important. In addition to these are known the rare 
compounds onofrite (Hg(S-Se)), tiemannite (HgSe) and color adoite 
(HgTe), all of which are isometric. 



98 DESCRIPTIVE MINERALOGY 

Cinnabar (HgS) 

Cinnabar is the only compound of mercury that occurs in sufficient 
quantity to constitute an important ore. Nearly all of the mercury, 
or quicksilver, in the world is obtained from it. The mineral occurs 
both crystallized and massive. The ore is a red crystalline mass that 
is easily distinguished from all other red minerals by its peculiar shade of 
color and its great weight. 

Theoretically, it contains 13.8 per cent S and 86.2 per cent Hg. 
Massive cinnabar is, however, usually impure through the admixture 
of clay, iron oxides or bituminous substances. Occasionally the quan- 
tity of organic material present is so large that the mixture is inflam- 
mable. 

Though cinnabar is usually granular, massive or earthy, it some- 
times occurs beautifully crystallized 
in small complex and highly modi- 
fied hexagonal crystals that exhibit 
tetartohedral forms (trigonal trape- 
zohedral class). Usually the crys- 
tals are rhombohedral or prismatic 
in habit. Their axial ratio is 
1 : I-I453- Planes belonging to 
more than 100 distinct forms have 
Fig, 38.-CiniK.bar Crystals wiih - R, been observed, but the crystals on 
10I0 (*); (R, 404s (-'); fR, aoi"s wnlcn thc y occur are usually so 
(ft; R, toil (r) and oR, 0001 (.-). small that few of them are of im- 
portance as distinguishing charac- 
teristics. The prismatic crystals, which are the most common in 
this country, are often bounded by 00 R, (10T0) and $R, (4045) 
(Fig. 38). Others, however, are very complicated. Their cleavage is 
perfect parallel to 00 R(ioTo). 

The mineral is slightly sectile. It is transparent, translucent or 
opaque, is of a cochineal-red color, often inclining to brown, and its 
streak is scarlet. Its hardness is only 2-2.5 an d * ls density about 
8.t. It is circularly polarizing and is a nonconductor of electricity. 
Its dimorph, metacinnabarite, on the other hand, is a good conductor. 
The indices of refraction of cinnabar are: 01= 2.854, (=3.201. 

When heated gently in the open tube cinnabar yields sulphurous 
fumes and globules of mercury. On charcoal before the blowpipe it 
volatilizes completely. 

There are only a few minerals with which cinnabar is likely to be 



SULPHIDES, TELLURIDES, ETC. 99 

confused, since its color and streak are so characteristic. From all 
red minerals but realgar it may easily be distinguished by its sulphur 
reaction. From realgar it is distinguished by its great density and its 
greater hardness. 

Pseudomorphs of cinnabar after stibnite, dolomite ((Ca-Mg)COa), 
pyrite and tetrahedrite (a complicated sulpho-salt) have been described. 

Synthesis. — Crystals have been made by heating mercury in an aque- 
ous solution of H2S. 

Occurrence, Localities and Origin. — Cinnabar is usually found in 
veins cutting serpentine, limestones, slates, shales and various schists. 
It is associated with gold, various sulphides, especially pyrite and mar- 
casite (FeS2), calcite (CaCOa), barite (BaSCU), fluorite (CaF2) and 
quartz. It is also found impregnating sandstones and other sedimen- 
tary rocks, and sometimes as a deposit from hot springs. Its deposi- 
tion is thought to be the result of precipitation from ascending hot 
water. 

Crystallized cinnabar occurs at a number of places in Bohemia, 
Hungary, Serbia, Austria, Spain, California, Texas, Nevada, and at 
other localities in Europe, Asia and South America. 

The principal deposits of economic importance are at Almaden 
in Spain, at Idria in the Province of Carniola, Austria, at Bakhmut 
in southern Russia, at various points along the Coast Ranges in Cal- 
ifornia, in Esmeralda, Humboldt, Nye and Washoe Counties in Nevada, 
at many points in Oregon and Utah, and at Terlingua in Texas. The 
mineral is also abundant in Peru and in China but in these countries 
it has not yet been mined profitably. The California cinnabar district 
extends for many miles along the Coast Ranges, but at only about a 
duzen places is the mineral mined. 

The Spanish mines, near the city of Cordova, have been worked 
for many hundreds of years. Much of the ore is an impregnation of 
sandstone and quartzite — the mineral sometimes comprising as much 
as 20 per cent of the rock mined. 

Extraction. — The metallurgy of cinnabar is exceedingly simple. It 
consists simply in roasting the ore alone, or mixed with limestone, and 
conducting the fumes into a condensing chamber that is kept cool. 
The sulphur gases are allowed to escape through the chamber in which 
the mercury is collected 

Uses of Metal. — Mercury finds many uses in the arts. Its most im- 
portant one is in the extraction of gold and silver by the amalgamation 
process. It is the essential constituent of the pigment vermilion, which 
is a manufactured HgS. In its metallic state it is largely employed in 



100 DESCRIPTIVE MINERALOGY 

the making of mirrors, of barometers, thermometers and other physical 
instruments. Some of the salts are important medicinal preparations 
while others are used in the manufacture of percussion caps. 

Production. — The world's annual production of quicksilver, all of 
which is obtained from cinnabar, is not far from 4,000 metric tons. The 
United States produced 940 tons in 1912, valued at $1,053,941. Of this 
total California yielded 20,524 flasks of 75 lbs. each, valued at about 
$863,034, and Texas and Nevada 4,540 flasks valued at $190,907. To 
produce these quantities of metal California mined 139,347 tons of ore 
and Texas and Nevada 16,346 tons. The California ore yielded 11 lbs. 
of metal per ton and the Nevada and Texas ore 20.8 lbs. 

Metacinnabarite (HgS) is generally found as a gray-black massive 
mineral with a black streak. It is brittle, has a hardness of 3 and a 
density of 7.8. It is associated with cinnabar at some of the mines in 
California and Mexico, and at a few places in other countries. It is 
exceedingly rare. 

THE METALLIC DISULPHIDES, DISELENIDES AND DIARSENIDES 

The disulphides, diselenides, ditellurides, diarsenides and dianti- 
monides differ from the corresponding monocompounds in that they 
contain double the quantity of S, Se, Te and Sb. They are divisible 
into two groups, one of which comprises sulphides, arsenides and anti- 
monides of iron, manganese, cobalt, nickel and platinum, and the other 
the tellurides and selenides of gold and silver, 

GLANZ GROUP 

The glanz group is an excellent illustration of an isodimorphous grbup. 
Its members are characterized by their hardness, opaqueness, light color 
and brilliant luster. Hence the name of the group. In composition 
the minerals belonging to the group are sulphides, arsenides or anti- 
monides of the iron-platinum group of metals, with the general formula 
RQ2 in which R is Mn, Fe, Ni, Co, Pt, and Q=S, As and Sb. The com- 
position of the more simple members may be represented by the formula 

Fe^ I , and of those in which arsenic or antimony replaces a part of the 

X S 

<As=Asv 
\Fe. 
S S X 

: It is probable, however, that some of the cobalt and nickel arsenides 



SULPHIDES, TELLURIDES, ETC. 



101 



are mixtures and that their indicated compositions are only approximate. 
All members of the group are believed to be dimorphous, crystallizing 
in the isometric (dyakisdodecahedral class), and in the orthorhombic 
systems (orthorhombic bipyramidal class), though not all have as yet 
been found in both forms. The most important members of the group, as 
at present constituted, are as follows: 



Isometric 




Orthorhombic 


Pyrite 


FeS2 


Marcasite 


Haueriie 


MnSs 






FeAsS 


Arsenopyrite 




FeAs2 


Ldllingite 


Cobaltite 


CoAsS 


Glancodot 


Gersdorfite 


(NiFe)AsS 




Korynite 


(Ni-Fe)(As-Sb-S) 2 


WolfackUe 


UUmanite 


NiSbS 




Smaltite 


C0AS2 


Safflorite 


CUoanthite 


NiAs2 


Rammelsbergite 


Sperrylite 


PtAS2 





The group is divided into two subgroups, the regularly crystallizing 
minerals forming the pyrite group and the orthorhombic ones the mar- 
casite group. The most important members of the former group are 
pyrite, cobaltite, smalttte and chloanthite. The most important members 
of the marcasite group are marcasite, arsenopyrite and loUingite. 

PYRITE DIVISION 

The crystallization of the pyrite group is in the parallel hemihedral 
division (dyakisdodecahedral class) of the isometric system. The 

[20ool 
, 210, is so frequently seen on the mineral 



occurrence of the form 



pyrite that it has received the name pyritoid. 

The group is so perfectly isomorphous that a description of the forms 
on one member is practically a description of the forms on all. 

Pyrite (FeS 2 ) 

Pyrite, one of the most common of all minerals, is found under a 
great variety of conditions as crystals, as crystalline aggregates and 
as crystalline masses. It occurs under practically all conditions and in 
all situations. It is easily recognized by its bright yellow color, its 
brilliant luster and its hardness. 



DESCRIPTIVE MINERALOGY 



Pyrite containing, theoretically, 46.6 per cent of iron and 53.4 per 
cent of sulphur is usually contaminated with small quantities of nickel, 



Fin. 39. — Group of Pyrite Crystals in which the Cube Predominates. The i_rystals 
are striated parallel to the edge between =0 O =0 (100) and l~— — I , i"°)- 



cobalt, thallium and other elements. An auriferous variety is worked 
for gold, yielding in the aggregate a large quantity of the precious 





Fig. 40- Fig. 41. 

FlC. 40.— Pyrite Crystals on which O (11 1) Predominates. o = 0, lit and e = ™Os 

Fig. 41.— Pyrite Crystal with nOi, 210(c) andO, m (a). 

metal. Sometimes arsenic is present in small quantity. Analysis of 
the crystals from French Creek, Penn., gave: 



S = 54.o8, As=o.20, Fe=44-24, Co = i.75, Ni=o.i8, Cu=o.os, =100.50. 



SULPHIDES, TELLURIDES, ETC. 103 
The number of forms that have been observed on pyrite crystals is 
very large. Hintze records 86. The cube and the pyritoid 



Fig. 43. — Group oi Pyrite Crystals in which o»02 (jio) Predominates. From 
Daly-Judge Mine, neat Part City, Utah. (After J. W. Boutwell.) 

(210) arc the most common of these, though the octahedron and the 

dodecahedron are not rare. Four distinct types of crystals may be 

recognized, viz.: those with the cubic (Fig. 3g), 

the octahedral (Fig. 40), and the pyritoid 

habits (Figs. 41 and 41), and those that are 

interpenetrating twins (Fig. 43). The cubic 

and the pyritoid planes are often striated 

parallel to the edges between these faces. The 

interpenetrating twins are twinned about the 

plane O(m). 

The cleavage of pyrite is imperfect and 
its fracture conchoids!. The mineral is 
brittle. Its hardness is 6-6.5 ar >d density 
about 5. Its luster is very brilliant and 
metallic. Its color is brassy yellow and its 
streak greenish or brownish black. With steel it strikes fire, hence its 
name from the Greek word meaning fire. It is a good conductor of 
electricity and is strongly thermo-electric. 




•"ig. 43. — Pyrite Interpene- 
Iration Twin. Two Pyri- 
tuids ( °° O-i, 216) Twinned 
about O, in. 



104 DESCRIPTIVE MINERALOGY 

In the closed tube pyrite yields a sublimate of sulphur and a residue 
that is magnetic. On charcoal sulphur is freed. This burns with the 
blue flame characteristic of the substance. The globule remaining after 
heating for some time is magnetic. Treated with nitric acid the 
mineral dissolves leaving a flocculent residue of sulphur, which when 
dried and heated may readily be ignited. 

Pyrite in some of its forms so closely resembles gold that it is often 
known as fool's gold. There is, of course, no difficulty in distinguishing 
between the two metals, since pyrite contains sulphur and is soluble in 
nitric acid,, while gold contains no sulphur and is insoluble in all simple 
acids. 

The mineral is most easily confounded with chalco pyrite (CuFeS2), 
though the difference in hardness of the two easily serves to distinguish 
them. Chalcopyrite may be readily scratched with a knife blade or a 
file, while pyrite resists both. The latter mineral, moreover, contains 
no copper. 

Syntheses. — Small crystals of pyrite are produced by the action 
of H2S on the oxides or the carbonate of iron enclosed in a sealed tube 
heated to 8o°-90°; also by the passage of H2S and FeCfe vapors through 
a red-hot porcelain tube. 

Occurrence and Origin. — Pyrite occurs in veins and as grains or 
crystals embedded in all kinds of rocks. In rocks it usually appears as 
crystals, but in vein-masses it may appear either as crystals, with other 
minerals, or as radiating or structureless masses occupying entirely the 
vein fissures. In slates it often occurs in spheroidal nodules and 
concretions of various forms, and also as embedded crystals. The 
mineral is the product of igneous, metamorphic and aqueous agencies. 

Pyrite weathers readily to limonite. In ore bodies near the 
surface it is oxidized. A portion of the mineral changes to FeS04 
which percolates downward and aids in the concentration of any 
valuable metals that may be present in small quantity in the ore. 
Another portion of the iron remains near the surface in the form of 
limonite. This covering of oxidized material is known as the " gossan " 
and it is characteristic of all pyrite deposits. 

Localities. — Pyrite crystals are so widely distributed that but very 
few of its most important occurrences may be mentioned here. In the 
mines of Cornwall, Eng., and in those on the Island of Elba very large 
crystals are found. Fine crystals also come from many different places 
in Bohemia, Hungary, Saxony, Peru, Norway, and Sweden. 

In the United States the finest crystals are at Schoharie and Rossie, 
N. Y.; at the French Creek mines in Chester Co., and at Cornwall, 



SULPHIDES, TELLURIDES, ETC. 105 

Lebanon Co., Penn., and near Greensboro and Guilford Co., N. Carolina. 
Massive pyrite occurs in great deposits at the Rio Tinto mines in 
Spain; at Rowe, Mass.; in St. Lawrence and Ulster counties, N. Y.; 
in Louise Co., Va., and in Paulding Co., Ga. Much of the massive 
pyrite in the veins of Colorado, California and of the southern states, 
from Virginia to Alabama, is auriferous and much of it is mined for the 
gold it contains. 

Uses. — Pyrite is used principally in the manufacture of sulphuric 
acid. The mineral is burned in furnaces and the SO2 gases thus result- 
ing are carried to condensers where they are oxidized by finely divided 
platinum or by the oxides of nitrogen. The residue, which consists 
largely of Fe203, is sometimes smelted for iron or made into paint. 
This residue also contains the gold and other valuable metals that may 
have been in the original pyrite. 

The sulphuric acid obtained from pyrite enters into many manu- 
facturing processes. The greater portion of it is consumed in the 
artificial fertilizer industry. 

Production. — Pyrite is mined in the United States in Franklin Co., 
Mass., in Alameda and Shasta Counties, California, in Louisa, Pulaski 
and Prince William Counties, Va., in Carroll Co., Ga., in St. Lawrence 
Co., N. Y., in Clay Co., Alabama, and at the coal mines in Ohio, 
Illinois and Indiana where it is a by-product. The total production 
of the United States in 191 2, amounting to 350,928 long tons, was 
valued at $1,334,259. Virginia is by far the largest producer. In 
addition to this quantity the trade consumed 970,785 tons of imported 
ore, most of which came from Spain, and utilized the equivalent of 
260,000 tons of pyrite in the shape of low grade sulphide copper ores 
from Ducktown, Tenn., and zinc sulphide concentrates from the Mis- 
sissippi Valley and elsewhere for the manufacture of sulphuric acid. 
The total amount of sulphuric acid manufactured in the United States 
during 191 2 was 2,340,000 short tons, valued at $18,338,019. The total 
world production of pyrite is about 2,000,000 tons annually. 

Small quantities of the mineral are also mined for local consumption 
in Lumpkin Co., Georgia, and near Hot Springs, Arkansas. Much 
auriferous pyrite has also been mined in the southern states and the 
Rocky Mountain region for the gold it contains. This metal is sepa- 
rated from the pyrite partly by crushing and amalgamation and partly 
by smelting or by leaching processes. In the former case the gold 
occurs as inclusions of the metal in the pyrite. 



106 DESCRIPTIVE MINERALOGY 



Cobaltite (CoAsS) 

Cobaltite is a silver-white or steel-gray mineral occurring in massive 
forms or in distinct crystals exhibiting beautifully their hemihedral 
character. It is completely isomorphous with the corresponding nickel 
compound, gersdorffite (NiAsS), and consequently mixtures of the 
two are common. 

Cobaltite usually contains some iron and often a little nickel. 
Theoretically, it consists of 19.3 per cent S, 45.2 per cent As and 35.5 
Co. The compositions of a massive variety from Siegen, Westphalia, 
and that of crystals from Nordmark, Norway, are as follows: 

As S Co Fe Ni Total 

Siegen 45.31 19.35 33-71 163 100.00 

Nordmark 4477 20.23 29.17 4.72 1.68 100.57 

The crystallization of cobaltite is perfectly isomorphous with that 
of pyrite, though the number of its forms observed is far smaller. The 

[2O 00 1 
(210). 

The cleavage of cobalt is fairly good parallel to 00 O 00 (100). Its 
fracture is uneven, its hardness is 5.5 and its density about 6.2. The color 
of the mineral, as stated above, varies between silver-white and steel- 
gray. Its streak is grayish black. It is a good conductor of electricity. 

In the open tube cobaltite reacts for S and As. On charcoal it 
yields a magnetic globule which when fused with borax on platinum 
wire yields a deep blue bead. It weathers fairly readily to the rose- 
colored cobalt arsenate known as erythrite (Co3(As04)2*8H20). 

By its crystallization and color cobaltite is distinguished from 
nearly all other minerals but those of the same group. From most of 
these it is easily distinguished by its blowpipe reactions for sulphur, 
arsenic and cobalt. 

Occurrence and Origin. — Cobaltite occurs mainly in veins that are 
believed to have been filled by upward moving solutions emanating 
from igneous rocks. It is associated with compounds of nickel and other 
cobalt compounds and with silver and copper ores. 

Localities. — Cobaltite is not very widely distributed. Large, hand- 
some crystals occur at Tunaberg in Sweden; at Nordmark, Norway; 
at Siegen, Westphalia, and near St. Just in Cornwall, England. It is 
found also in large quantity at Cobalt, Ontario, associated with silver 
ores and nickel compounds. 



SULPHIDES, TELLURIDES, ETC. 107 

Uses. — Cobaltite is said to be used by jewelers in India in the pro- 
duction of a blue enamel on gold ornaments. It is employed also in the 
manufacture of blue and green pigments and in the manufacture of com- 
pounds used in small quantity in the various arts. Smalt is the most 
valuable of the cobalt pigments and is at present the chief commercial 
compound of this metal. It is a deep blue glass that differs from 
ordinary glass in containing cobalt in place of calcium. Smalt is hiade 
from cobaltite and from other cobalt ores by fusion with a mixture of 
quartz and potassium carbonate. Certain cobalt compounds are sug- 
gested as excellent driers for oils and varnishes. The mineral is also 
utilized as an ore of cobalt, which in the form of stellite, an alloy com- 
posed of 70 per cent cobalt, 15 per cent chromium and 15 per cent 
molybdenum or tungsten, bids fair to acquire a large use as a material 
for the manufacture of table cutlery and edged tools. The use of the 
metal has also been suggested as a material for coinage in place of 
nickel. 

Production. — Most of the cobalt of commerce is handled by the 
trade in the form of the oxide. It is produced from the various cobalt 
minerals, mainly as a by-product in the extraction of nickel, and hence 
very little is obtained from cobaltite. The mines at Cobalt, however, 
have furnished a large quantity of cobaltite and smaltite within the past 
few years and these have gone into the manufacture of the oxide, of 
which about 515 tons were produced in 191 2, having a value of 
$317*165. 

Smaltite (CoAs 2 ) 

Smaltite is another important ore of cobalt. It is found in crystals 
and masses. 

Its theoretical composition is 71.88 per cent As and 28.12 per cent 
Co, though it usually contains also S, Ni, Fe and frequently traces of 
Bi, Cu and Pb. Since it is isomorphous with the arsenide of nickel 
chloanthite (N1AS2), mixed crystals of the two are common. Moreover, 
sharply defined crystals have been found to consist of mechanical mix- 
tures of several compounds. 

Smaltite occurs in small crystals of cubical habit with 00 O 00 (100), 
O(in) and various pyritoids predominating. 

The mineral is tin-white to steel-gray, and opaque, and has a grayish 
black streak. It is often covered with an iridescent or a gray tarnish. 
Its cleavage is indistinct, its fracture uneven, its hardness 5-6 and 
density 6.3-7. It is a good electrical conductor. 

Before the blowpipe on charcoal smaltite yields arsenic fumes and a 



108 DESCRIPTIVE MINERALOGY 

magnetic globule of metallic cobalt. It is soluble in HNO3, yielding a 
rose-colored solution and a precipitate of AS2O3. 

The mineral is fairly easily distinguished from most other minerals 
by its color and blowpipe reactions. From cobaltite it is distinguished by 
the lack of S. From a few others that are not described in this volume 
it can be distinguished by its crystallization or by quantitative analysis. 

Synthesis. — Smaltite crystals are produced when hydrogen acts at a 
high temperature upon a mixture of the chlorides of cobalt and arsenic. 

Occurrence and Origin. — Smaltite is found associated with cobaltite 
in nearly all of its occurrences. It is especially abundant at Cobalt, Ont. 
As in the case of most other cobalt minerals, its presence is indicated by 
deposits of rose-colored erythrite which coat its surfaces wherever these 
are exposed to moist air. Its methods of occurrence, origin and uses 
are the same as for cobaltite (p. 107). 

Chloanthite (NiAs2) resembles smaltite in most of its characteris- 
tics. The two minerals grade into each other through isomorphous 
mixtures. Those mixtures in which the cobalt arsenide is in excess 
are known as smaltite, while those in which NiAs predominates are 
called chloanthite. The pure chloanthite molecule is Ni= 28.1 per cent, 
As =71.9 per cent. 

The two minerals can be distinguished when unmixed with one 
another by the blowpipe reactions for Co and Ni. In mixed crystals 
the predominance of one or the other arsenides can be determined only 
by quantitative analysis. 

Chloanthite containing much iron is distinguished as chathamite, 
from Chatham, Conn., where it occurs with arsenopyrite and niccolite in 
a mica-slate. 

The mode of occurrence of chloanthite and the localities at which 
it is found are the same as in the case of smaltite. 

Sperrylite (PtAs 2 ) 

Sperrylite is extremely rare. It is referred to here because it is the 
only platinum compound occurring as a mineral. Chemically, it is 
43.53 per cent As and 56.47 per cent Pt, but it contains also small quan- 
tities of Sb, Pd and Fe. 

Its crystals are simple. They contain only O(in), 00 O 00 (100), 
00 0(i 10) and several pyritoids. Their habit is usually octahedral or 
cubical. 

The mineral is opaque and tin-white, and its streak black. Its hard- 
ness is 6-7 and density 10.6. 



SULPHIDES, TELLURIDES, ETC. 109 

In the closed glass tube it remains unchanged, but in the open tube 
it gives a sublimate of AS2O3. When dropped upon red-hot platinum 
foil it immediately melts, giving rise to fumes of AS2O3, and forming 
blisters on the foil that are not distinguishable from the original platinum 
in color or general character. It is slowly soluble in concentrated HC1 
and aqua regia. 

Synthesis. — The mineral has been produced by leading arsenic fumes 
over red-hot platinum in an atmosphere of hydrogen. 

Occurrence and Localities. — Sperrylite occurs as little crystals com- 
pletely embedded in the chalcopyrite (CuFeS2) and the gossan of a 
nickel mine, and in the chalcopyrite of a gold-quartz vein near Sudbury, 
Ontario; in covellite at the Rambler Mine, Encampment, Wyoming; 
and as flakes in the sands of streams in the Cowee Valley, Macon Co., Ga. 
The flakes resemble very closely native platinum, from which they are 
of course, easily distinguished by the test for arsenic. 

Uses. — The sperrylite from Sudbury and Wyoming furnish much of 
the platinum produced in the United States (see p. 64). 

MARCASITE DIVISION 

Three members of the marcasite group are important; all are inter- 
esting from the fact that they are so alike in their crystallization that a 
description of the forms belonging to any one of them might serve as a 
description of those belonging to all others. The crystallization of the 
group is orthorhombic (rhombic bipyramidal class), with an axial ratio 
approximately a : b : c=.y : 1 : 1.2. 

m 

Marcasite (FeS 2 ) 

Marcasite, the dimorph of pyrite, resembles this mineral so dosely 
that in massive specimens it is difficult to distinguish between the two. 
They are nearly alike in hardness, in color and in chemical properties. 
Marcasite is a little lighter in color than pyrite. Its density is less 
(about 4.9), and it possesses a greater tendency to tarnish on exposed 
surfaces. 

This tarnish indicates that the mineral is more susceptible to altera- 
tion than is pyrite. One of the products of this alteration is ferrous sul- 
phate, which may often be detected by its taste upon touching the tongue 
to specimens of the mineral. In crystallized specimens there is not the 
least difficulty in distinguishing between them, since their crystallization 
is very different. 

Marcasite is orthorhombic (rhombic bipyramidal class), with the 



110 DESCRIPTIVE MINERALOGY 

axial ratio .7662 : 1 : 1.2342. Its simple crystals often possess a tabular 
or a pyramidal habit (Figs. 44 and 45). In the former case oP(ooi) is 
the predominant face, and in the latter case the two domes P 66 (101) 





Fie. 45. 



Fie. 44.— Mucuite Crystal with «P,no(m); oP, 001 (e); PS, ,011 (0 and JPB , 

013 (»). 

Fig. 45. — Marcasite Crystal with Forms as Indicated in Fig. 44, and P » , 101 («) 

andP, in (s). 

and P 06 (on). The other forms observed on most crystals are 00 P(no), 
P(iii), and often jP * (013). 

Twins are very common, with 00 P(i 10) the twinning plane (Fig. 46). 
Sometimes these are aggregated by repeated twinning into serrated 
groups known as cockscomb twins or spearhead twins (Fig. 47), because 




Fie. 46. 

Fig. 46. — Twin of Marcasite about » 



of the outlines of their edges. In many instances the crystals are acic- 
ular or columnar in habit, forming radiating groups with globular, reni- 
form and stalactitk shapes. Concretions are also common. The basal 
plane is usually striated parallel to the edge between it and P06 (on). 
The cleavage is distinct parallel to 00 P(no). The fracture is uneven 



SULPHIDES, TELLURIDES, ETC. Ill 

When powdered marcasite is treated with cold nitric acid and 
allowed to stand, it decomposes with the separation of sulphur. 

Marcasite readily alters to limonite. The fact that pyrite, sphaler- 
ite, chalcopyrite, and other minerals form pseudomorphs after it 
indicates that, under suitable conditions, it alters also to these com- 
pounds. The mineral is in most cases a direct result of precipitation 
from hot solutions. 

Synthesis. — Marcasite crystals have been prepared by the reduction 
of FeS04 by charcoal in an atmosphere of HaS. 

Occurrence and Uses. — The mineral, like pyrite, is found embedded 
in rocks in the form of crystals and concretions, and also as the 
gangue masses of veins. It constitutes nearly the entire filling of some 
veins, and forms druses on the walls of cavities in both rocks and miner- 
als. It also replaces the organic matter of fossils preserving their shapes 
— thus producing true pseudomorphs. 

When associated with pyrite it is mined together with this mineral 
as a source of sulphur. 

Localities. — Crystalline marcasite occurs in such great quantity 
near Carlsbad in Bohemia that it is mined. The cockscomb variety is 
found in Derbyshire, England, and crystals at Schemnitz in Hungary 
and at Andreasberg and other places in the Harz. In the United States 
the mineral occurs as crystals at a great number of places, being par- 
ticularly abundant in the lead and zinc localities of the Mississippi 
Valley, where it sometimes forms stalactites. The stalactites from 
Galena, 111., often consist of concentric layers of sphalerite, galena and 
crystallized marcasite. 

Arsenopyrite (FeAsS) 

Arsenopyrite, or mispickel, is the most important ore of arsenic. 
It is found in crystals and in compact ar.d granul&r masses. It is a 
silver-white metallic mineral resembling very closely cobaltite in its 
general appearance. 

The formula FeAsS for arsenopyrite is based on analyses like the 
following: 

As S Fe Total 

Specimen from Hohenstein, Saxony .... 45 . 62 19 . 76 34 . 64 100 . 02 
Specimen from Mte. Chalanches, France 45 .78 19 . 56 34 .64 99 . 98 

Theoretically, the mineral consists of its components in the following 
proportions, As 46 per cent, S 19.7 per cent, Fe 34.3 per cent. In many 
specimens the iron is replaced in part by cobalt, nickel or manganese. 



112 



DESCRIPTIVE MINERALOGY 



lw m m I 




Fig. 48. — Arsenopyrite Crystals with 00 P, 
no (m); JP 00 , 014 (m), and P 00 , on (q). 



Sometimes the cobalt is present in such large quantity that the mineral 
is smelted as an ore of this metal. 

The axial ratio of arsenopyrite is .6773 : * : 1-1882. Its crystals are 
usually simpler than those of marcasite (Fig. 48), though the number of 
planes observed in the species is larger. Most of the untwinned crystals 

are a combination of 00 P(no) 
with £P 06 (014), or P 06 (on), 
or Px>(ioi), and have a pris- 
matic habit. Twins are not 
rare. The twinning plane is 
the same as in marcasite, 
and repetition is often met 
with. The angle iioai^o= 
68° 13'. 

The brachydomes are stri- 
ated horizontally, and often 
the planes ooP(no) are stri- 
ated parallel to the edge 00 P(iio)aP* (ioi). 

The cleavage of arsenopyrite is quite perfect parallel to ooP(no). 
The mineral is brittle and its fracture uneven. Its hardness is 5.5-6 
and density about 6.2. Its color is silver- white to steel-gray; its streak 
grayish black. It is a good conductor of electricity. 

In the closed tube arsenopyrite at first gives a red sublimate of AsS 
and then a black mirror of arsenic. On charcoal it gives the usual 
reactions for sulphur and arsenic. Cobaltiferous varieties react for 
cobalt with borax. The mineral yields sparks when struck with steel 
and emits an arsenic smell. It dissolves in nitric acid with the separa- 
tion of sulphur. 

Arsenopyrite is distinguished from the cobalt sulphides and arsenides 
by the absence of Co. 

Synthesis. — Crystals of the mineral are produced by heating in a 
closed tube at 300 precipitated FeAsS in a solution of NaHCC>3. 

Occurrence. — Arsenopyrite crystals are often found disseminated 
through crystalline rocks, and often embedded in the gangue minerals of 
veins. Like pyrite and marcasite they frequently fill vein fissures. Its 
associates are silver, tin and lead ores, chalcopyrite, pyrite and sphalerite. 
Localities. — The mineral is abundant at Freiberg, in Saxony, at 
Tunaberg, in Sweden, and at Inquisivi Mt., Sorato, in Bolivia. 

It also occurs in fine crystals at Franconia in New Hampshire, at 
Blue Hill in Maine, at Chatham in Connecticut, and at St. Francois, 
Beauce Co., Quebec. Massive arsenopyrite is found near Keeseville 



SULPHIDES, TELLURIDES, ETC. 113 

Essex Co., near Edenville, Orange Co., and near Carmel, Putnam Co., 
N. Y., and at Rewald, Floyd Co., Va. In most cases it is apparently 
a result of pneumatolysis. 

Uses. — Arsenopyrite was formerly the source of nearly all the arsenic 
of commerce. The mineral is concentrated by mechanical methods, and 
the concentrates are heated in retorts, when the following reaction takes 
place: FeAsS = FeS+As. The arsenic being volatile is conducted 
into condensing chambers where it is collected. When the mineral con- 
tains a reasonable amount of cobalt or of gold these metals are extracted. 

Uses of Arsenic. — The metal arsenic has very little use in the arts, 
though its compounds find many applications as insecticides, medicines, 
pigments, in tanning, etc. The basis of most of these is AS2O3, and 
this is produced directly from the fumes of smelters working on arsenical 
gold, silver and copper ores. Only a portion of such fumes are saved, 
however, as even half of those produced at a single smelter center 
(Butte, Montana), would more than supply the entire demand of the 
United States for arsenic and its compounds. Under these conditions 
the mining of arsenical pyrite as a source of arsenic has ceased so far 
as the United States is concerned. 

Lollingite (FeAso) is usually massive, though its rare crystals are 
isomorphous in every respect with those of arsenopyrite. The pure 
mineral is not common. Most specimens are mixtures of ldllingite with 
arsenopyrite or other sulphides or arsenides. 

The mineral is silver-white or steel-gray. Its streak is grayish black. 
Its hardness is 5-5.5 and density about 7.2. It readily fuses to a mag- 
netic globule, at the same time evolving arsenic fumes. It is soluble in 

HNO3. 

It usually occurs in veins associated with other sulphides and arsen- 
ides. It is found at Paris, Maine; at Edenville and Monroe, N. Y.; 
at various mines in North Carolina, and on Brush Creek, Gunnison 
Co., Colo. At the last-named locality the mineral is in star-shaped 
crystalline aggregates, in twins and trillings, associated with siderite 
and barite. 

SYLVANITE GROUP 

The sylvanite group includes at least three distinct minerals, all of 
which are ditellurides of gold or silver. The group is isodimorphous. 
The pure gold telluride is known only in monoclinic crystals, but the 
isomorphous mixtures of the gold and silver compounds occur both in 
monoclinic and orthorhombic crystals. 



114 DESCRIPTIVE MINERALOGY 

i 

Orthorhombic bipyramidal Monoclinic prismatic 

AuTe2 Calaverite 

Krenneriie (Ag • Au)Te2 SylvaniU 

All three minerals are utilized as ores of gold. While occurring only 
in a few places, they are sufficiently abundant at some to be mined. 

Calaverite (AuTe 2 ) 

Calaverite is a nearly pure gold chloride. However, it is usually 
intermixed with small quantities of the silver telluride. An analysis of a 
specimen from Kalgoorlie, Australia, gave: Te = 57.27; Au = 4i.37; 
Ag=.s8. 

Calaverite crystallizes in the monoclinic system (prismatic class) in 
crystals that are elongated parallel to the orthoaxis and deeply striated 
in this direction. Their axial ratio is 1.6313 : 1 : 1.1440 with £=90° 13'. 

The prominent forms are 00 P 06 (100), 00 P 00 (010), oP(ooi), 
— P*(ioi), +Pob(ioT), — 2P«x)(20i), +2P«>(2oT), and P(in). 
Twinning is common and the resulting twins are very complicated. 
Usually, however, the mineral occurs massive and granular. 

Calaverite is opaque, silver-white or bronzy yellow in color and has a 
yellow-gray or greenish gray streak. Its surface is frequently covered 
with a yellow tarnish. The mineral is brittle and without distinct cleav- 
age. Its hardness is 2-3 and density 9.04. 

On charcoal before the blowpipe the mineral fuses easily to a yellow 
globule of gold, yielding at the same time the fumes of tellurium oxide. 
It dissolves in concentrated H2SO4, producing a deep red solution. When 
treated with HNO3 it decomposes, leaving a rusty mass of spongy gold. 
The solution treated with HC1 usually yields a slight precipitate of silver 
chloride. 

Calaverite is distinguished from most other minerals by the test for 
tellurium. It is distinguished from fetzite (p. 80), by its crystallization 
and the fact that it gives a yellow globule when roasted on charcoal, 
and from sylvanite by the small amount of silver it contains, its higher 
specific gravity, its color and its lack of cleavage. It is distinguished 
from krenneriie by its crystallization. 

Occurrence. — The mineral occurs in veins with the other tellurides 
associated with gold ores in Calaveras Co., Cal., and at the localities 
mentioned for petzite (see p. 81). It is believed to have been deposited 
by pneumatolytic processes or by ascending magmatic water at com- 
paratively low temperatures. 



SULPHIDES, TELLURIDES, ETC. 115 

Uses. — The mineral is mined with other tellurides in Boulder Co., 
and at Cripple Creek, Colorado, as an ore of gold. 

Sylvanite (Ag-Au)Te 2 

Sylvanite is more common than calaverite. It is an isomorphous 
mixture of gold and silver tellurides in the ratio of about i : i. Analyses 
follow: 

I. Te=62.i6 Au=24.45 Ag—13.39 Total=ioo.oo 

II. Te=59.78 Au=26.36 Ag=i3.86 " 100.00 

III. Te=58.9i Au=29«35 Ag=ii-74 " 100.00 

I." Theoretical for AgTea+AuTe,. 
II. and III. Specimens from Boulder Co., Colo. 

In crystallization the mineral is isomorphous with calaverite, with 
an axial ratio a : b : c— 1.6339 : 1 : 1.1265 and 0=90° 25'. Its crystals 
are usually rich in planes, about 75 having been identified. Their habit 
is usually tabular parallel to 00 P ob (010), with this plane, — P «> (101), 
oP(ooi), 00 P 6b (100) and 2P2(?2i) predominating. The mineral also 
occurs in skeleton crystals and in aggregates that are piaty or granular. 
Twinning is common, with — P*(ioi) the twinning plane. Many 
twinned aggregates form networks suggesting writing, hence the name 
" Schrifterz " often applied to the mineral by the Germans. 

Sylvanite is silver-white or steel-gray and has a brilliant metallic 
luster and a silver-white or yellowish gray streak. Its hardness is 
between 1 and 2 and its density 7.9-8.3. Moreover, it possesses a per- 
fect cleavage parallel to ooPw (010). 

Its chemical properties are the same as those of calaverite, but the 
silver precipitate produced by adding HC1 to its solution in HNO3 is 
always large. It is best distinguished from the gold telluride by its 
cleavage and from fetzite ((Ag.Au)2Te) and hessite (Ag2Te) by its 
crystallization, and by the yellow metallic globule produced when the 
mineral is roasted on charcoal. It is distinguishable from krennerite by 
its crystallization. 

Localities and Origin, — Sylvanite occurs with the other tellurides in 
veins at Offenbdnya and Nagyag in Transylvania, at Cripple Creek and 
in Boulder Co., Colo., near Kalgoorlie, W. Australia, in small quan- 
tities near Balmoral in the Black Hills, S.D., and at Moss, near Thunder 
Bay, Ontario. Like calaverite it was deposited by magmatic water, or 
by hot vapors. 

Uses. — It is mined with calaverite as a gold and silver ore at Cripple 
Creek and in Boulder Co., Colo. 



CHAPTER V 
THE SULPHO-SALTS AND SULPHO-FERRITES 

The sulpho-salts are salts of acids analogous to arsenic acid, H3ASO4, 
and arsenious acid, H3ASO3, and the corresponding antimony acids 
H3SDO4 and H3SDO3. The sulpho-acids differ from the arsenic and the 
antimony acids in containing sulphur in place of oxygen, thus H3ASS4, 
H3ASS3, H3SDS4 and H3SDS3. The mineral enargite may be regarded as 
a salt of sulpharsenic acid, thus C113ASS4, copper having replaced the 
hydrogen of the acid. Proustite, on the other hand, is AgaAsS3, or a, 
salt of sulpharsenious acid. The salts of sulpharsenic acid are called 
sulpharsenates, while those derived from sulpharsenious acid are known 
as sulpharsenites. The sulpharsenates are not represented among the 
commoner minerals, although the copper salt enargite is abundant at a 
few places. A number of salts of other sulphur-arsenic acids are known 
but they are comparatively rare. 

There is another class of compounds with compositions analogous 
to those of the sulpho-salts, though their chemical nature is not well 
understood. This is the group of the sulpho-ferrites. We know that 
certain hydroxides of iron may act as acids under certain conditions. 
The sulpho-ferrites may be looked upon as salts of these acids in which, 
however, the oxygen has been replaced by sulphur, as in the case of the 
sulpho-acids referred to above. Thus by replacement of O by S, in 
ferric hydroxide Fe(OH)3 the compound Fe(SH;3 or IfcFeSs results. 
The salts of this acid are sulpho-ferrites. This acid, by loss of H2S, 
may give rise to other acids in the same way that sulphuric acid (H2SO4), 
by loss of H2O, gives rise to pyrosulphuric acid. In the case of the 
sulpho-acid we may have H3FeS3— H2S = HFeS2. The copper salt of 
this acid is the common mineral chalcopyrite, CuFeS2- 

The sulpho-salts are very numerous, but only a few of them are of 
sufficient importance to warrant a description in this book. 



116 



SULPHO-SALTS AND SULPHO-FERRITES 117 



THE SULPHARSENITES AND SULPHANTTMONITES 

The sulpharsenites and sulphantimonites are derivatives of the 
ortho acids H3ASS3 and H3SbS3. 



ORTHO SULPHO-SALTS 

The ortho salts are compounds in which the hydrogen of the ortho 
acids is replaced by metals. They include a large number of minerals, 
of which the following are the most important. 

Bournonite (Cu2 ■ Pb)3 (SbSs)2 Orthorhombic 

Pyrargyrite Ag3SbS3 Hexagonal 

ProuslUe Ag3AsS3 Hexagonal 

PYRARGYRITE GROUP 

Pyrargyrite (Ag3SbSs) 

Pyrargyrite, or dark ruby silver, is an important silver ore, especially 
in Mexico, Chile and the western United States. The name ruby silver 
is given to it because thin splinters transmit deep red light. The mineral 
is usually mixed with other ores in compact masses, but it also forms 
handsome crystals. 

The composition of pyrargyrite is represented by the formula Ag3SbS3 
which demands 17.82 per cent S.; 22.21 per cent Sb.; 59.97 per cent Ag. 
Many specimens contain also a small quantity of arsenic, through the 
admixture of the isomorphous compound proustite. The analyses given 
below show the effect of the intermixture of the two molecules. 



- 


S 


Sb 


As 


Ag 


Total 


Andreasberg, Harz . . . 


• 1765 


22.36 


• • • ■ 


59- 73 


99-77 


Zacatecas, Mexico 


• 1774 


22.39 


■ 2 7 


60.04 


100.44 


Freiberg, Saxony 


• 17-95 


18.58 


2.62 


60.63 


99.78 



The crystals of pyrargyrite are rhombohedral and hemimorphic 
(ditrigonal pyramidal class), with an axial ratio 1 : .8038. They are 
usually quite complex and are often twinned. The species is very rich 
in forms, not less than 150 having been reported. The most prominent 
of these are ooP2(ii2o), ooP(ioTo), R(ioTi), — ^R(oil2) and the 
scalenohedrons R 3 (2i3i) and JR 3 (2I34) (Fig. 49). In the commonest 
twinning law the twinning plane is 00 P2 (11 20) and the composition 



118 



DESCRIPTIVE MINERALOGY 





Fig. 49. — Crystal of 
Pyrargyritc with 
00 P2, 1 120 (a) 
and — JR,oil2(«). 



face oP(ooi). The c axes in the twinned portions are parallel and the 
00 P2 (11 20) planes coincident, so that the twin at a hasty glance looks 
like a simple crystal. The angle 10T1 A ^101 = 71° 22'. 

The cleavage of pyrargyrite is distinct parallel to R(ioTi). Its frac- 
ture is conchoidal or uneven. The mineral is apparently opaque and its 

color is grayish black in reflected light, but is trans- 
parent or translucent and deep red in transmitted 
light. Its streak is purplish red. For lithium 
light o>= 3.084, €=2.881. It is not an electrical 
conductor. 

In the closed tube the mineral fuses easily and 
gives a reddish sublimate. When heated with 
sodium carbonate on charcoal it is reduced to a 
globule of silver, which, when dissolved in nitric 
acid, yields a silver chloride precipitate when 
treated with a soluble chloride. The mineral dis- 
solves in nitric acid with the separation of sulphur 
and a white precipitate of antimony oxide. It is also soluble in a 
strong solution of KOH. From this solution HC1 precipitates orange 
Sb2S3 (compare proustite). 

The color and streak of pyrargyrite, together with its translucency, 
distinguish it from nearly all other minerals. Its reaction for silver 
serves to distinguish it from cuprite, cinna.ar and realgar, which it some- 
times resembles. The distinction between this mineral and its iso- 
morph, proustite, is based on the streak and the reaction for anti- 
mony. 

Pyrargyrite occurs as a pseudomorph after native silver. On the 
other hand it is occasionally altered to pyrite or argentite, and some- 
times to silver. 

Syntheses. — Microscopic crystals have been made by heating in a 
porcelain tube, metallic silver and antimony chlorides in a current of 
H2S, and by the action of the same gas at a red heat on a mixture of 
metallic silver and melted antimony oxide. 

Occurrence, Localities and Origin. — Pyrargyrite occurs in veins asso- 
ciated with other compounds of silver and sometimes with galena and 
arsenic. It is most common in the zone of secondary enrichment of 
silver veins. The crystallized variety is found at Andreasberg in the 
Harz; at Freiberg, in Saxony; at Pribram, in Bohemia; at many places 
in Hungary, and at Chafiarcillo, in Chile. The massive variety is worked 
as an ore of silver at Guanajuato in Mexico and in several of the western 
states, as, for instance, in the Ruby district, Gunnison Co.. and in other 



SULPHO-SALTS AND SULPHO-FERRITES 119 

mining districts in Colorado, near Washoe and Austin, Nevada, and at 
several points in Idaho, New Mexico, Utah and Arizona. 

Uses. — The mineral is an important ore of silver in Mexico and in 
the western United States. It is usually associated with other sulphur- 
bearing ores of silver, the metal being extracted from the mixture by 
the processes referred to under argentite. 

Proustite (Ag3AsS 3 ) 

Proustite, or light ruby silver, is isomorphous with pyrargyrite. It 
differs from the latter mineral in containing arsenic in place of antimony. 
It occurs both massive and in crystals, and like pyrargyrite is an ore of 
silver. 

The formula above given demands 1943 per cent S; 15.17 per cent 
As; and 65.40 per cent silver.. The analysis of a specimen from Mexico 
yields figures that correspond very nearly to these. Crystals from 
Chafiarcillo contain a slight admixture of the antimony compound. 

S 

Mexico 19 . 52 

Chafiarcillo, Chile 19-64 

Like pyrargyrite, proustite is rhombohedral. Its crystals are pris- 
matic or acute rhombohedral. The forms present on them are much 
less numerous than those on the corresponding 
antimony compound, the predominant ones being 
ooP 2 (ii2o), JR(ioT4), -JR(oii2), R 3 (2i3i), 
—|R 4 (3587) and other scalenohedrons (see Fig. 
50). Twins are common, the twinning planes 
being (1), parallel to }R(iol4) and (2) parallel to 

R(ioTi). The angle 10T1 aTici = 72° 12'. FlG - 5o.-Crystal of 

mi 1 £ -l j i_ j £ Proustite with 00 P2. 

The cleavage, fracture and haidness of prous- - , . #T>1 ' 

£ t, i_ j II2 ° W» ~! R » 35S7 

tite are the same as for pyrargyrite. Its hard- ^ M ) and -JR,oii2 (e). 

ness is 2 and its density is about 5.6. The mineral 

is transparent or translucent. Its color is grayish black by reflected 

light arid scarlet in transparent pieces by transmitted light. Under the 

long-continued influence of daylight the color deepens until it becomes 

darker than that of pyrargyrite. Its streak is cinnabar-red to brownish 

black. Its luster is adamantine. It is a nonconductor of electricity. 

For sodium light 01=3.0877, €= 2.7924. 

In the closed tube proustite fuses easily and gives a slight sublimate 



As 


Sb 


Ag 


Total 


14.98 


• • • ■ 


65-39 


99.89 


I3-85 


I. 41 


65.06 


99.96 




120 DESCRIPTIVE MINERALOGY 

of white arsenic oxide. In its other chemical properties it resembles 
pyrargyrite except that it gives reactions for arsenic where this mineral 
reacts for antimony, and yields only sulphur when dissolved in HNO3. 
From its solution in KOH a yellow precipitate of AS2S3 is thrown down 
upon the addition of HC1 (compare pyrargyrite). 

Proustite differs from pyra.gyite in its color, transparency and 
streak, as well as in its arsenic reactions. It is distinguished from 
cinnabar and cuprite (CuO) by the arsenic test. 

Syntheses. — Crystals of proustite have been produced by reactions 
analogous to those that yield pyrargyrite, when arsenic compounds are 
employed in place of antimony compounds. 

Occurrence. — The mineral occurs under the same conditions and with 
the same associates as pyrargyrite and it yields the same alteration 
products as pyrargyrite. 

Localities and Uses. — Handsome crystals of proustite occur at 
Freiberg and other places in Saxony, at Wolfach in Baden, at Markirchen 
in Alsace and at Chaftarcillo in Chile. It is associated with pyrargyrite 
and with other ores of silver. 

In the western United States it is quite abundant, more particularly 
in the Ruby district, Colorado, at Poorman lode in Idaho, and in all other 
localities where pyrargyrite occurs. In many it is mined as an ore of 
silver. 

Bournonite ((Pb-Cu 2 )3(SbS 3 )2) 

Bournonite is a comparatively rare mineral. It occurs either in 
compact or granular masses or in well developed crystals of a steel- 
gray color. It is not of any economic importance except as it may be 
mixed with other copper compounds exploited for copper. 

Analyses of bournonite from two localities are given below: 

S Sb 

I. I9-3 6 23.57 
II. 19.78 23.80 

I. Liskeard, Cornwall, England. 

II. Felsdbanya, Hungary. 

These analyses are by no means accurate, but they show the compo- 
sition of the mineral to be approximately Pb, Cu, Sb and S, in which the 
elements are combined in the following proportions: S=ig.8 per cent; 
Sb= 247 per cent; Pb 42.5 per cent; Cu 13 per cent. 

Bournonite crystals are orthorhombic (rhombic bipyramidal class), 



As 


Pb 


Cu 


Fe 


Total 


•47 


41-95 


1327 


.68 


99 30 


• • • 


42.07 


12.82 


.20 


98.67 



SULPHO-SALTS AND SULPHO-FERRITES 



121 



with alb: £=.9380 : 1 : .8969. They are usually tabular (Fig. 51), or 
short, prismatic in habit, and are often in repeated twins (Fig. 52), with 
wheel-shaped or cross-like forms. The principal planes observed on 
them are oP(ooi), Poo (ioi),Poo (on), £P(ii2), ooP(no), 00 Poo (100) 
and 00 P 06 (010), though 00 or more planes are known. The most com- 
mon twinning plane is 00 P(no). Angle no A iTo=86° 20'. 

The luster of the mineral is brilliant metallic. Its color and streak 
are steel-gray. Its cleavage is imperfect, parallel to 00 P 06 (010) and its 
fracture conchoidal or uneven. Its hardness is 2.5-3 an d density 5.8. 
Like most other metallic minerals it is opaque. It is a very poor con- 
ductor of electricity. 

In the closed tube bournonite decrepitates and yields a dark red sub- 
limate. In the open tube, and on charcoal, it gives reactions for Sb, S, 
Pb and Cu. When treated with nitric acid it decomposes, producing a 








V 


**l 




■^ 




< 


*~^ 


a 




b 


m 


& 


m 





Fig. 51. Fig. 52. 

Fig. 51. — Bournonite Crystal with oP, 001 (r); Poo , 101 (0); JP, 112 (u) and Poo , 

on (»). 
Fig. 52. — Bournonite Fourling Twinned about 00 p, no (m). Form c same as in 

Fig. 51. 6= 00 P 00 (010) and a— 00 P 65 (100). 

blue solution of copper nitrate that turns to an intense azure blue when 
an excess of ammonia is added. In this solution is a residue of sulphur 
and a white precipitate that contains lead and antimony. 

Bournonite is distinguished from most other minerals by its reactions 
for both antimony and sulphur. From other sulphantimonites it is 
distinguished by its color, hardness and density. 

On long exposure to the atmosphere bournonite alters to the car- 
bonates of lead (cerussite) and copper (malachite and azurite). 

Synthesis, — Crystals of bournonite have been obtained by the action 
of gaseous H2S on the chlorides and oxides of Pb, Cu and Sb, at moderate 
temperatures. 

Occurrence. — The mineral occurs principally in veins with galena, 
sphalerite, stibnite, chalcopyrite and tetrahedrite. 

Localities. — Good crystals are found in the mines at Neudorf, Harz; 
at Pribram, in Bohemia; at Felsobanya, Kapnik and other places 
in Hungary, and at various places in Chile. In North America it has 



122 DESCRIPTIVE MINERALOGY 

been found at the Boggs Mine in Yavapai Co., Ariz., in Montgomery 
Co., Ark., and at Marmora, Hastings Co., and Darling, Lanark Co., 
Ontario. 

THE SULPHDIARSENITES AND SULPHDIANTIMONITES 

A large number of sulpho-salts we apparently salts of acids that 
contain two or more atoms of As or Sb in the molecule. These acids 
may be regarded as derived from the ortho acids by the abstraction of 
H2S, thus: The arsenious acid containing two atoms of As may be 
thought of as 2H3ASS3— H2S = H4As2S6. Acids with larger proportions 
of arsenic may be regarded as derived in a similar manner from three or 
more molecules of the ortho acid. Only a few of these salts are common 
as minerals. Among the more common are two that are lead salts of 
derivatives of sulpharsenious and sulphantimonous acids. 

Jamesonite (Pb 2 Sb 2 S 5 ) and Dufrenoysite (Pb 2 As 2 S 5 ) 

Jamesonite and dufrenoysite are lead salts of the acids H4SD2S5 and 
H4AS2S5. Both minerals occur in acicular and columnar orthorhombic 
crystals and in fibrous and compact masses of lead-gray color. Their 
cleavage is parallel to the base. The minerals are brittle and have an 
uneven to conchoidal fracture. Their hardness is 2-3 and density 
5.5-6. The streak of jamesonite is grayish black, and of dufreynosite 
reddish brown. Both minerals are easily fusible. They are soluble in 
HO with the evolution of H2S, giving a solution from which acicular 
crystals of PbCk separate on cooling. They are decomposed by HNO3, 
with the separation of a white basic lead salt. They are found in veins 
with antimony and sulphide ores abroad and at several points in Nevada, 
and in the antimony mines in Sevier Co., Arkansas. 

THE SULPHARSENATES AND SULPHANTIMONATES 

The sulpharsenates are salts of sulpharsenic acid, H3ASS4, and the 
sulphantimonates, the salts of the corresponding antimony acid, HaSbS4. 
These compounds are much less numerous among the minerals than the 
sulpharsenites and sulphantimonites. Moreover, no member of the 
former groups is as common as several of the members of the latter. 
The most important member is the mineral enargite (C113ASS4) an ortho- 
sulpharsenate, which in a few places is wrought as a copper ore. 



SULPHO-SALTS AND SULPHO-FERRITES 



123 



Enargite (Cu 3 AsS 4 ) 

Enargite, though a rare mineral, is so abundant at a few points that 
it has been mined as an ore of copper. 

Theoretically, the mineral is S = 32.6, As=i9.i, 01=4.83. Most 
specimens, however, contain an admixture of the isomorphous anti- 
mony compound, famatinite, and consequently show the presence of 
antimony. A specimen from the Rams Mine, Butte, Montana, yielded 



s 


As 


Sb 


Cu 


Fe 


Zn 


Ins 


Total 


3 J -44 


17.91 


1.76 


48.67 


•33 


.10 


.11 


100.32 




Fig. 53. — Enargite Crys- 
tal with 00 P, no (m); 
oopoo , 100(a); 00 P^, 
1 20 (A) and oP, 001 (c). 



The mineral crystallizes in the orthorhombic system (bipyramidal 
class), in crystals with an axial ratio .8694 : 1 : .8308. Their habit is 
usually prismatic, and they are strongly striated 
vertically. The crystals are usually highly modi- 
fied, with the following forms predominating: 
00 P56 (100), ooP(no), ooP2(i2o), ooP3(i3o), 
00 P 06 (010), and oP(ooi) (Fig. 53). Stellar trill- 
ings, with ooP2(i2o) the twinning plane, have a 
pseudohexagonal habit. The mineral occurs also 
in columnar and platy masses. 

Enargite possesses a perfect prismatic cleavage 
and an uneven fracture. It is opaque with a 
grayish black color and streak. Its hardness is 3 
and density 4.4. It is a poor electrical conductor. 

It is easily fusible before the blowpipe. When roasted on charcoal 
it gives the reactions for S and As, and thq roasted residue when 
moistened with HC1 imparts to the flame the azure-blue color char- 
acteristic of copper. In the closed tube it decrepitates and gives a 
sublimate of S. When heated to fusion it yields a sublimate of arsenic 
sulphide. The mineral is soluble in aqua regia. 

Enargite is easily recognized by its crystallization and blowpipe 
reactions. 

Occuf fence. Tr-Eneirgite is associated with other copper ores in veins 
filled by magmatic water at intermediate depths and in a few replace- 
ment deposits. 

Localities. — Although not widely distributed, enargite occurs in large 
quantities in the copper mines near Morococha, Peru; Copiapo, Chile; 
in the province of La Rioja, Argentine; on Luzon, Philippine Islands, 



124 DESCRIPTIVE MINERALOGY 

and in the United States, at Butte, Montana; in the San Juan Moun- 
tains, Colorado, and in the Tintic District, Utah. 

Uses. — It is smelted as an ore of copper. At the Butte smelter it 
furnishes the arsenic that is separated from the smelter fumes and placed 
upon the market as arsenic oxide (see p. 113). 

THE BASIC SULPHO-SALTS 

The basic sulpho-salts are compounds in which there is a greater 
percentage of the basic elements (metals, etc.), present than is 
necessary to replace all the hydrogen of the ortho acids. Thus, the 
copper orthosulpharsenate, enargite, is CU3ASS4 The mineral steph- 
anite is AgsSbS4 and the pure silver polybasite AggSbSo. 

Since three atoms of Ag are sufficient to replace all the hydrogen 
atoms in the normal acid containing one atom of antimony and the 
quantities of silver present in stephanite and polybasite are in excess 
of this requirement, the two minerals are described as basic. The 
exact relations of the atoms to one another in the molecules are 
not known. 

Although the number of basic sulpho-salts occurring as minerals is 
large only four arc common. These are: 

StepJtanite AgoSbS4 Orthorhombic 

Polybasite (Ag-Cu)9SbSo Monoclinic 

Tetrahedrite (R")4Sb2S7 Isometric 

Tennantite (R")4As2S7 Isometric 

Stephanite (Ag 5 SbS 4 ) 

Stephanite, though a comparatively rare mineral, is an important ore 
of silver in some camps. It occurs massive, in disseminated grains and 
as aggregates of small crystals. Analyses indicate a composition very 
close to the requirements of the formula AgsSbS4. 

S Sb Ag AsandCu Total 

Theoretical 16.28 15.22 68.50 .... 100.00 

Crystals, Chaiiarcillo, Chile 16.02 15.22 68 . 65 tr 99 . 89 

Stephanite crystallizes in hemimorphic orthorhombic crystals (rhom- 
bic pyramidal class), with an axial ratio .6291 : 1 : .6851. The crystals 
are highly modified, 125 forms having been identified upon them. They 
have usually the habit of hexagonal prisms, their predominant planes 




SULPHO-SALTS AND SULPHO-FERRITES 125 

being ooP(no) and ooP 06(010), terminated by oP(ooi), P(in) and 
2 P 06 (021) at one or the other end of the c axis (Fig. 54). Twins are 
common, with 00 P(no) and oP(ooi) the twinning planes. 

The mineral is black and opaque and its streak is black. Its hard- 
ness is 2 and density =6. 2 — 6.3. It cleaves 
parallel to 00 P 06 (010) has an uneven frac- 
ture, and is a poor conductor of electricity. 

On charcoal stephanite fuses very easily 
to a dark gray globule, at the same time 
yielding the white fumes of antimony oxide Fig. 54.— Stephanite Crystal 
and the pungent odor of SO2. Under the with oP, 001 (c); 00 p«, 

reducing flame the globule is reduced to OI ° )** °°- S 1IC ,i\ ' 

.. 332 (P), 2P00 , 021 (d). 

metallic silver. The mineral dissolves in 

dilute nitric acid and this solution gives a white precipitate with HC1. 

Stephanite is easily distinguished from other black minerals by its 
easy fusibility, its crystallization, and its reactions for Ag, Sb and S. 

Localities. — The mineral is associated with ether silver ores in the 
zone of secondary enrichment of veins at Freiberg, Saxony; Joachimsthal 
and Pribram, Bohemia; the Comstock Lode and other mines in the 
Rocky Mountain region and at many points in Mexico and Peru. 

Uses. — It is mined together with other compounds as an ore of silver^ 
It is particularly abundant in the ores of the Comstock Lode, Nev., and 
of the Las Chispas Mine, Sonora, Mex. 

Polybasite ((Ag-Cu) 9 SbS 6 ) 

Polybasite is the name usually applied to the mixture of basic sulph- 
antimonites and suipharsenites of the general formula R'9(Sb.As)Se, in 
which R' = Ag and Cu. More properly the name is applied to the anti- 
monite, and the corresponding arsenite is designated as pearceiie. Sev- 
eral typical analyses follow: 

Fe Pb Ins Total 

.... ... ... 99 ' "9 

I.05 ... .42 99.85 

.... ... ... 1UO . V/U 

76 ... 100.18 

I. Pearceite, Veta Rica Mine, Sierra Mojada, Mexico. 
II. Crystals of pearceite, Drumlummon Mine, Marysville, Montana. 

III. Polybasite, Santa Lucia Mine, Guanajuato, Mexico. 

IV. Polybasite, Quespisiza, Chile. 





s 


As 


Sb Ag 


Cu 


I. 


17.46 


7-56 


59-22 


1565 


II. 


17.71 


7-39 


55.17 


18. n 


III. 


15.43 


■50 


10.64 68.39 


5i3 


IV. 


16.37 


388 


515 67.95 


6.07 



126 DESCRIPTIVE MINERALOGY 

The crystallization of the two minerals, which are completely isomor- 
phous, is monoclinic (prismatic class). Their axial ratios are: 

Pearceite, a : b : c= 1.7309 : 1 : 1.6199 £=90° 9' 
Polybasite, =17309 : 1 : 1.5796 £=90° 

The crystals are commonly tabular or prismatic, with a distinct 
hexagonal habit. The prominent forms are oP(ooi), P(in) and 
2P 06 (20T). Contact twinning is common, with 00 P(no) the twinning 
plane, and oP(ooT) the composition plane. 

Both minerals are nearly opaque. Except in very thin splinters 
they are steel-gray to iron-black in color. Very thin plates are trans- 
lucent and cherry-red. Their streaks are black. Their cleavage is 
perfect parallel to oP(ooi) and their fracture uneven. Their hardness 
is 2-3, and density 6-6.2. 

Both minerals are easily fusible. They usually exhibit the reactions 
for Ag, Sb, As and S. 

They are readily distinguished from all other minerals but silver 
sulpho-salts by their blowpipe reactions. From these they are distin- 
guished by their crystallization. Pearceite and polybasite are distin- 
guished from one another by the relative quantities of As and Sb they 
contain. 

Occurrence. — Both minerals occur in the zone of secondary enrich- 
ment in veins of silver sulphides. 

Localities. — Polybasite was an important ore of silver in the Comstock 
Lode, Nevada. It is at present mined with other silver ores at Ouray, 
Colorado, at Marysville, Montana, at Guanajuato, Mexico, and at 
various points in Chile. Good crystals occur at Freiberg* Saxony, at 
Joachimsthal, Bohemia, and in the mines in Colorado, Mexico and Chile. 

TETRAHEDRITE GROUP 

The name tetrahedrite is given to a mixture of basic sulphanti- 
monites and sulpharsenites crystallizing together in isometric forms with 
a distinct tetrahedral habit (hextetrahedral class). The isomorphism 
is so complete that all gradations between the various members of the 
group are frequently met with. The arsenic-bearing member of the 
series is known as tennantite and the corresponding antimony member as 
tetrahedrite. The latter is the more common. 

The following six analyses of tetrahedrite will give some idea of the 
great range in composition observed in the species. 



SULPHO-SALTS AND SULPHO-FERRITES 127 

S Sb As Cu Fe Zn Ag Hg Pb Total 

I. 27.60 25.87 tr 35.85 2.665.15 2.30 99.43 

II. 23.51 17.21 7.67 42.00 8.28 .49 .55 99.71 

III. 24.44 27.60 .... 27.41 4.27 2.31 14.54 IOO S7 

IV. 24.8930.18 tr 32.80 5.85 07 5.57 99.36 

V. 21.67 24.72 .... 33.53 .56 1.80 16.23 98.51 

I. Newburyport, Mass. 
II. Cajabamba, Peru. 

III. Star City, Nev. 

IV. Pontes, Hungary. 
V. Arizona. 

Upon examination these are found to correspond approximately to 
the formula R"4Sb2S7, in which the R" is CU2, Pb, Fe, Zn, Hg, Ag2 and 
sometimes Co and Ni. When R is replaced entirely by copper, the 
formula (CugSbaSz) demands 23.1 per cent S, 24.8 per cent Sb and 52.1 
per cent Cu. 

Analyses of tennantite yield analogous results that may be repre- 
sented by the formula CU8AS2S7 which demands 26.6 per cent S, 20.76 
per cent As and 52.64 per cent Cu. 

Analyses of even the best crystallized specimens rarely yield As or 
Sb alone. Moreover, nearly all show the presence of Zn in notable 
quantity. The great variation noted in the composition of different 
specimens which appear to be pure crystals has led to the proposal of 
other formulas than those given above — some being simpler and others 
more complex. It is possible that the variation may be explained as 
due, in part, to some kind of solid solution, rather than as the result 
solely of isomorphous replacement. It is more probable, however, that 
it is due to the intergrowth of notable quantities of various sulphides 
with the sulpho-salts. 

There is still considerable confusion in the proper naming of the mem- 
bers of the series, but generally the forms composed predominantly of 
Cu, Sb and S with or without Zn are known as tetrahedrite and those 
containing As in place of Sb as tennantite, although several authors 
confine the use of the latter term to arsenical tetrahedrites containing a 
notable quantity of iron. 

Since the members of the tetrahedrite series often contain a large 

* 

quantity of metals other than Cu and Zn the group has been so sub- 
divided as to indicate this fact. Thus, there are argentiferous, mercurial 
and plumbiferous varieties of tetrahedrite. Some of these varieties are 
utilized as ores of the metals that replace the copper and zinc in the more 



Pb 


Co 


Total 


.11 


• • • • 


100.15 


• • • 


• • • B 


99.21 


• • • 


I. 20 


101.07 



128 DESCRIPTIVE MINERALOGY 

common varieties. The relations of the ordinary (II) and the bis- 
muthif erous tennantites (III) to tetrahedrite (I) are shown by the fol- 
lowing three analyses: 

S As Sb Bi Cu Fe Ag 

I. 24.48 tr 28.85 45-39 I -3 2 

II. 26.61 19.03 51.62 1.95 

III. 29.10 11.44 2.19 13.07 37.52 6.51 .04 

I. Fresney d'Oisans, France. 
II. Cornwall, England. 
III. Cremenz, Switzerland. 

The crystals of both tetrahedrite and tennantite are tetrahedral in 

habit, the principal forms on them consisting of the simple tetrahedron 

2O2 |0 _ 

and complex tetrahedrons such as (211), (332) together with 

2 2 

the dodecahedron, ooO(no) and the cube, 00 O 00 (100). (Fig. 55.) 

Twins are common with O(ni) the twinning 

plane. These are sometimes contact twins 

and sometimes interpenetration twins. Some 

crystals are very complicated, because of the 

presence on them of a great number of forms. 

The total number of distinct forms that have 

been identified is about 90. The mineral 

_, occurs also in granular, dense and earthy 
Fig. <«>. — Tetrahedrite Crys- 

masses, 

tal with - in (0); 00 o, The fracture of the tetrahedrites is uneven, 
no (d) and s O, 332 (»). Their hardness varies between 3 and 4.5 and 

their density between 4.4 and 5.1. Their color 
is between dark gray and iron-black, except in thin splinters, which 
sometimes exhibit a cherry-red translucency. Their streak is like their 
color. All tetrahedrites are thermo-electric. 

The chemical properties of the different varieties of tetrahedrites 
vary with the constituents present. All give tests for sulphur and for 
either antimony or arsenic, and all show the presence of copper in a 
borax bead. The reactions of other metals that may be present may 
be learned by consulting pages 483-494. 

The crystals of tetrahedrite are so characteristic that there is little 
danger of confusing the crystallized mineral with other minerals of the 
same color. The massive forms resemble most clearly arsenopyrite, 
coba'tiU, bournonite and chalcociie. From these the tetrahedrites are 




SULPHO-SALTS AND SULPHO-FERRITES 129 

best distinguished by their hardness, together with their blowpipe reac- 
tions. 

Tetrahedrite appears to suffer alteration quite readily, since pseudo- 
morpbs of several carbonates and sulphides after tetrahedrite crystals 
are well known. 

Syntheses. — Crystals of the tetrahedrites have been made by passing 
the vapors of the chlorides of the metals and the chlorides of arsenic or 
antimony and H2S through red-hot porcelain tubes. They have also 
been observed in Roman coins that had lain for a long time in the hot 
springs of Bourbonne-les-Bains, Haute-Marne, France. 

Occurrence. — The tetrahedrites are very common in the zone of 
secondary enrichment of sulphide veins and in impregnations. They 
occur associated with chalcopyrite, pyrite, sphalerite, galena and other 
silver, lead and copper ores in nearly all regions where the sulphide ores 
of these metals are found. They occur also as primary constituents of 
veins of silver ores, where they were deposited by magmatic waters. 

Localities. — In the United States tetrahedrite occurs at the Kellogg 
Mines, ten miles north of Little Rock, Arkansas; near Central City and 
at Georgetown, Colorado; in the Ruby and other mining districts in 
the same State; at the De Soto Mine in Humboldt Co., Nevada, and 
at several places in Montana, Utah and Arizona. It is found also in 
British Columbia and in Mexico, and at Broken Hill, New South Wales. 

The arsenical tetrahedrites are not quite as common as is the anti- 
monial variety. Excellent crystals occur in the Cornish Mines, at 
Freiberg in Saxony, at Skutterud in Norway, and at Capelton, 
Quebec. 

Uses. — The mineral is used to some extent as an ore of silver or of 
copper, the separation of the metals being effected in the same way as 
in the case of the sulphides of these substances. 

THE SULPHO-FERRITES 

Only two sulpho-ferrites are sufficiently important to merit descrip- 
tion here. Both of these are copper compounds and both are used as 
ores of this metal, one — chalcopyrite — being one of the most important 
ores of the metal at present worked. 

The first of these minerals discussed, bornite, is a basic salt of 
the acid HaFeSa; the second is the salt of the derived acid HFeS2, 
which may be regarded as the normal acid from which one molecule of 
H2S has been abstracted (see p. 116). 



130 DESCRIPTIVE MINERALOGY 

Bornite (Cu 5 FeS 4 ) 

Bornite, known also as horseflesh ore because of its peculiar purplish- 
red color, is found usually massive. In Montana and in Chile it con- 
stitutes an important ore of copper. 

Bornite is probably a basic sulpho-ferrite, though analyses yield 
results that vary quite widely, especially in the case of massive varieties. 
This variation is due to the greater or less admixture of copper sulphides, 
mainly chalcocite, with the bornite. The theoretical composition of the 
mineral is 25.55 S, 63.27 Cu, and 1 1.18 Fe. The analyses of a crystallized 
variety from Bristol, Conn., and of a massive variety from the Bruce 
Mines, Ontario, follow: 

S Cu Fe Ins Total 

Bristol, Conn 25.54 63.24 11.20 ... 99.98 

Bruce Mines, Ont 2 S-39 62.78 11.28 .30 99.75 

The crystallization of bornite is isometric (hexoctahedral class), in 
combinations of 00 O 00 (ico), 00 0(no),0(in), and sometimes 202(211). 
Crystals often form interpenetration twins, with O the twinning plane. 

The fracture of the mineral is conchoidal, its hardness 3 and density 
about 5. On fresh fracture the color varies from a copper-red to a pur- 
plish brown. Upon exposure alteration rapidly takes place covering 
the mineral with an iridescent purple tarnish. Its streak is grayish 
black. It is a good conductor of electricity. 

Chemically, the mineral possesses no characteristics other than those 
to be expected from a compound of iron, copper and sulphur. It dis- 
solves in nitric acid with the separation of sulphur. 

It is easily recognized by its purplish brown color on fresh fractures 
and its purple tarnish. 

Bornite alters to chalcopyrite, chalcocite, covellite, cuprite (CU2O), 
chrysocolla (CuSi03 • 2H2O) and the carbonates, malachite and azurite. 
On the other hand, bornite pseudomorphs after chalcopyrite and chal- 
cocite are not uncommon. 

Syntheses. — Roman copper coins found immersed in the water of 
warm springs in France have been partly changed to bornite. Crystals 
have been formed by the action of H2S at a comparatively low tempera- 
ture (ioo°-2oo° C), upon a mixture of CU2O, CuO and Fe203 

Occurrence and Origin. — Bornite is usually associated with other 
copper ores in veins and lodes, where it is in some cases a primary min- 
eral deposited by magmatic waters and in others a secondary mineral 
produced in the zone of enrichment of sulphide veins. It also sometimes 



SULPHO-SALTS AND SULPHO-FERRITES 131 

impregnates sedimentary rocks, where its origin is part due to contact 
action. 

Localities. — The crystallized mineral occurs near Redruth, Cornwall, 
Eng., and at Bristol, Conn. The massive mineral is found at many 
places in Norway and Sweden. It is the principal ore of some of the 
Bolivian, Chilian, Peruvian and Mexican mines and of the Canadian 
mines near Quebec. In the United States it has been mined at 
Bristol, Conn., and at Butte, Montana, 

Uses. — Bornite is mined with chalcopyrite and other copper com- 
pounds as an ore of this metal. 

Chalcopyrite (CuFeS 2 ) 

From an economic point of view this mineral is the most important 
of the sulpho-salts, as it is one of the most important ores of copper 





Fig. 56. Fig. 5;. 

Fig. 56.— Chalcopyrite Crystal with P, 111 (J>); -P, 
Fig. 57.— Chalcopyrite Crystal with — , 772 (*) and 

sometimes approaches w I'(iio) and x approach! 

Fig. 58.— Chalcopyrite Twinned about P(m}. 

known. It occurs both massive and crystallized. From its similarity 
to pynte in appearance it is often known as copper pyrites. 

Crystallized specimens of chalcopyrite contain 35 per cent S, 34.5 
per cent Cu and 30.5 per cent Fe, corresponding to the formula CuFeSa, 
i.e., a copper salt of the acid HFeSj. The mineral often contains small 
quantities of intermixed pyrite. It also contains in some instances 
selenium, thallium, gold and silver. 

The crystallization of chalcopyrite is in the sphenoidal, hemihedral 
division of the tetragonal system (tetragonal scalenohedron class). 



132 DESCRIPTIVE MINERALOGY 

p 

The crystals are usually sphenoidal in habit with the sphenoids -(in), 

2 
|P 

and — (332) the predominant forms (Figs. 56 and 57). In addition to 
2 

these there are often present also 00 P 00 (100), 00 P(no), 2P 00 (201), 

xp 
and a very acute sphenoid that is approximately —(772), supposed to be 

2 

p 

due to the oscillation of 00 P(no) and —(in) (Fig. 57). Twins are quite 

common, with the twinning plane parallel to P (Fig. 58). The plus 
faces of the sphenoid are often rough and striated, while the minus faces 
are smooth and even. 

The fracture of the mineral is uneven. Its hardness is 3.5-4 and 
density about 4.2. Its luster is metallic and color brass-yellow. Old 
fracture surfaces are often tarnished with an iridescent coating. Its 
streak is greenish black. It is an excellent conductor of electricity. 

On charcoal the mineral melts to a magnetic globule. When mixed 
with Na2C03 and fused on charcoal, a copper globule containing iron 
results. When treated with nitric acid it dissolves, forming a green 
solution in which float spongy masses of sulphur. The addition of 
ammonia to the solution changes it to a deep blue color and at the same 
time causes a precipitate of red ferric hydroxide. 

From the few brassy colored minerals that resemble it, chalcopyrite 
is distinguished by its hardness and streak. 

W"hen subjected to the action of the atmosphere or to percolating 
atmospheric water chalcopyrite loses its iron component and changes 
to covellite and chalcocite. The iron passes into limonite. Bornite, 
copper and pyrite are also frequent products of its alteration. In the 
oxidation zone of veins it yields limonite, the carbonates, malachite and 
azurite, and cuprite (CU2O). When exposed to the leaching action of 
water, limonite alone may remain to mark the outcrop of veins, the 
copper being carried downward in solution to enrich the lower portions 
of the vein. The deposit of limonite on the surface is known as 
" gossan." 

Syntheses. — Crystals of chalcopyrite have been produced by the 
action of H2S upon a moderately heated mixture of CuO and Fe203 
enclosed in a glass tube. The mineral has also been made by the action 
of warm spring waters upon ancient copper coins. It is also a fairly 
common product of roasting-oven operations. ' 

Occurrence and Origin. — Chalcopyrite is widely disseminated as a 
primary vein mineral, and is often found in nests in crystalline rocks. 



SULPHO-SALTS AND SULPHO-FERRITES 133 

It also impregnates slates and other sedimentary rocks, schists and 
altered igneous rocks where, in some cases, it is a contact deposit 
and in others is original. It is also formed by secondary processes caus- 
ing enrichment of copper sulphide veins. Its most common associ- 
ates are galena, sphalerite and pyrite» It is the principal copper ore 
in the Cornwall mines, where it is associated with cassiterite (SnCfe), 
galena and other sulphides. It is also the important copper ore of 
the deposits of Falun, Sweden, of Namaqualand in South Africa, 
those near Copiap6 in Chile, those of Mansfeld, Germany, of the Rio 
Tinto district in Spain, of Butte and other places in Montana, and of 
the great copper-producing districts in Arizona, Utah and Nevada. 

Crystals occur near Rossie, Wurtzboro and Edenville, N. Y., at the 
French Creek Mines, Chester Co., Perm., near Finksburg, Md., and at 
many other places. 

Extraction. — The mineral is concentrated by mechanical methods. 
The concentrates are roasted at a moderately high temperature, the iron 
being transformed into oxides and the copper partly into oxide and 
partly into sulphide. Upon further heating with a flux the iron oxide 
unites with this to form a slag and the copper sulphide melts, and collects 
at the bottom of the furnace as " matte," which consists of mixed copper 
and copper sulphide. This is roasted in a current of air to free it from 
sulphur. By this process all of the copper is transformed into the oxide, 
which may be converted into the metal by reduction. The metal is 
finally refined by electrical processes. Much of the copper obtained 
from chalcopyrite contains silver or gold, or both, which may be recov- 
ered by any one of several processes. 

Uses. — A large portion of the ccpper produced in the world is obtained 
by the smelting of chalcopyrite and the ores associated with it. 

Production. — The world's total product of copper has been referred 
to in another place (p. 55). Of this total (2,251,300,000 lb.) the United 
States supplied, in 191 2, 1,243,300,000 lb., of which about 1,000,000,000 
lb. were obtained from sulphide ores. Arizona and Montana produced 
the greater portion of this large quantity, the former contributing about 
359,000,000 lb. to the aggregate, and the latter 308,800,000 lb. Out- 
side of the United States the most important copper-producing countries 
are Mexico, Japan, Spain and Portugal, Australia, Chile, Canada, 
Russia, Peru and Germany, in the order named. Practically all of this 
copper, except that from Japan and Mexico, is extracted from sulphide 
ores. 



CHAPTER VI 

THE CHLORIDES. BROMIDES, IODIDES AND FLUORIDES 

The salts belonging to this group are compounds of metals with 
hydrochloric (HC1), hydrobromic (HBr), hydriodic (HI) and hydro- 
fluoric (HF) acids. Only a few are of importance. Of these some are 
simple chlorides, others are simple fluorides, others are double chlorides 
or fluorides (i.e. cryolite, AlF3+3NaF), and others are double hydrox- 
ides and chlorides (atacamite). 

THE CHLORIDES 

The simple chlorides crystallize in the isometric system, but in differ- 
ent classes in this system. They comprise salts of the alkalies, K, Na 
and NH4, and of silver. Of these only three minerals are of importance, 
viz.: sylvite, halite and cerargyrite. 

Halite (NaCl) 

Halite, or common salt, is the best known and most abundant of the 
native chlorides. It is a colorless, transparent mineral occurring in 
crystals, and in granular and compact masses. 

Pure halite consists of 39.4 per cent CI and 60.6 per cent Na. The 
mineral usually contains as impurities clay, sulphates and organic 
substances. The several analyses quoted below indicate the nature of 
the commonest impurities and their abundance in typical specimens. 

NaCl CaCl MgCl CaS0 4 Na 2 S0 4 Mg2S0 4 Clay H 2 

23 3° 

2.00 .70 

■33 



*•' 97 • 35 .... .... 


I. Ol 


•43 


II. 90 . 3 .... .... 


S°° 


2.00 


III. 98.88 tr tr 


•79 


• . • • 


I. Stassfurt, Germany. 






II. Vic, France. 






III. Petit Arise, La. 







.... •*.. 



The crystallization of halite is isometric (hexoctahedral class), the 
principal forms being 00 O 00 (100), O(in) and ooO(no). Often the 

134 




CHLORIDES, FLUORIDES, ETC. 135 

faces of the forms are hollowed or depressed giving rise to what are called 
" hopper crystals " (Fig. 59). The mineral occurs also in coarse, gran- 
ular aggregates, in lamellar and fibrous masses and in stalactites. 

Its cleavage is perfect parallel to 00 O 00 (100). Its fracture is con- 
choidal. Its hardness is 2-2.5 an d density about 2.17. Halite, when 
pure, is colorless, but the impurities present often color it red, gray, 
yellow or blue. The bright blue mottlings observed in 
many specimens are thought to be due to the presence 
of colloidal sodium. The mineral is transparent or 
translucent and its luster is vitreous. Its streak is 
colorless. Its saline taste is well known. It is 
diathermous and is a nonconductor of electricity. p IG SQ .— Hopper- 
The mineral is plastic under pressure; and its plasticity Shaped Cube of 
increases with the temperature. Its index of refraction Halite, 
for sodium light, n— 1.5442. 

In the closed tube halite fuses and often it decrepitates. When 
heated before the blowpipe it fuses (at 776 ) and colors the flame yellow. 
The chlorine reaction is easily obtained by adding a small particle of the 
mineral to a microcosmic salt bead that has been saturated with copper 
oxide. This, when heated before the blowpipe, colors the flame a bril- 
liant blue. The mineral easily dissolves in water, and its solution yields 
an abundant white precipitate with silver nitrate. 

The solubility of halite is accountable for a large number of 
pseudomorphs. The crystals embedded in clays are gradually dissolved, 
leaving a mold that may be filled by other substances, which thus 
become pseudomorphs. 

Syntheses. — Crystals of halite have been produced by sublimation 
from the gases of furnaces, and by crystallization from solution contain- 
ing sodium chloride. 

Occurrence and Origin. — Salt occurs most abundantly in the water of 
the ocean, of certain salt lakes, of brines buried deep within the rocks in 
some places, and as beds interstratified with sedimentary rocks. In the 
latter case it is associated with sylvite (KC1), anhydrite (CaS04), gypsum 
(CaS04-2H20), etc., which, like the halite, are believed to have been 
formed by the drying up of salt lakes or of portions of the ocean that 
were cut off from the main body of water, since the order of occurrence 
of the various beds is the same as the order. of deposition of the corre- 
sponding salts when precipitated by the evaporation of sea water at 
varying temperatures. (Comp. pp. 22, 23.) 

Below are given figures showing the composition of the salts in the 
water of the ocean, of Great Salt Lake, and of the Syracuse, N. Y., and 



136 DESCRIPTIVE MINERALOGY 

Michigan artificial brines (produced by forcing water to the buried rock 
salt). 

NaCl CaCl 2 MgCl 2 NaBr KC1 Na 2 S0 4 K 2 S0 4 CaS0 4 MgS0 4 

I. 77.07 .... 7.86 1.30 3.89 4.63 5.29 

II. 7957 .... 10.00 6.25 3.60 .58 

III. 9597 .90 .69 2 54 

IV. 91.95 3 .19 2 48 2 *39 

I. Atlantic Ocean. 
II. Great Salt Lake. 

III. New York brines. 

IV. Michigan brines. 

Localities. — The principal mines of halite, or rock salt, are at Wie- 
lic2ka, Poland; Hall, Tyrol; Stassfurt, Germany, where fine crystals 
are found; the Valley of Cardova, Spain; in Cheshire, England and in 
the Punjab region of India. At Petit Anse in Louisiana, in the vicinity 
of Syracuse, N. Y., and in the lower peninsula of Michigan thick beds 
of the salt are buried in the rocks far beneath the surface. Much of the 
salt is comparatively pure and needs only to be crushed to become usable. 
In most cases, however, it is contaminated with clay and other sub- 
stances. In these cases it must be dissolved in water and recrystallized 
before it is sufficiently pure for commercial uses. 

The best known deposits are at Stassfurt where there is a great thick- 
ness of alternating layers of halite, sylvite (KC1), anhydrite, gypsum, 
kieserite (MgS0 4 -H 2 0) and various double chlorides and sulphates of 
potassium and magnesium. Although the halite is in far greater quan- 
tity than the other salts, nevertheless, the deposit owes most of its value 
to the latter, especially the potassium salts (comp. pp. 137, 142). 

Uses. — Besides its use in curing meat and fish, salt is employed in 
glazing pottery, in enameling, in metallurgical processes, for clearing 
oleomargarine, making butter and in the more familiar household oper- 
ations. It is also the chief source of sodium compounds. 

Production. — Most of the salt produced in the United States is ob- 
tained directly from rock salt layers by mining or by a process of solu- 
tion, in which water is forced down into the buried deposit and then to 
the surface as brine, which is later evaporated by solar or by artificial 
heat. In the district of Syracuse, N. Y., salt occurs in thick lenses 
interbedded with soft shales. In eastern Michigan and in Kansas salt 
is obtained from buried beds of rock salt, and in Louisiana from great 
dome-like plugs covered by sand, clay and gravel. Some of the masses 
in this State are 1,756 ft. thick. 



CHLORIDES, FLUORIDES, ETC. 137 

The salt production of the United States for 191 2 amounted to 33,- 
324,000 barrels of 280 lb. each, valued at $9,402,772. Of this quantity 
7,091,000 barrels were rock salt. 

The imports of all grades of salt during the same time were about 
1,000,000 barrels and the exports about 440,000 barrels. 

Sylvite (KC1) 

Sylvite is isometric, like halite, but the etched figures that may be 
produced on the faces of its crystals indicate a gyroidal symmetry (pen- 
tagonal icositetrahedral class). The habit of the crystals is cubic with 
0(i 11) and 00 O 00 (100) predominating. 

Pure sylvite contains 47.6 per cent CI and 52.4 per cent K, but the 
mineral usually contains some NaCl and often some of the alkaline sul- 
phates. 

The physical properties of sylvite are like those of halite, except that 
its hardness is 2 and the density 1.99. Its melting temperatuie is 738 
and n for sodium light = 1.4903. 

When heated before the blowpipe the mineral imparts a violet tinge 
to the flame, which can be detected when masked by the yellow flame of 
sodium by viewing it through blue glass. Otherwise sylvite and halite 
react similarly. 

Halite and sylvite are distinguished from other soluble minerals by 
the reaction with the bead saturated with copper oxide, and from one 
another by the color imparted to the blowpipe flame. 

Synthesis. — Sylvite crystals have been made by methods analogous 
to those employed in syntheses of halite crystals. 

Occurrence. — Sylvite occurs associated with halite, but in distinct 
beds, at Stassfurt, Germany, and at Kalusz. Galicia. It has also been 
found, together with the sodium compound, incrusting the lavas of 
Vesuvius. 

Uses. — Sylvite is an important source of potassium salts, large quan- 
tities of which are used in the manufacture of fertilizers. 

CERARGYRITE GROUP 

The cerargyrite group comprises the chloride, bromide and iodide of 
silver. The first two exist as the minerals cerargyrite and bromargyrite, 
both of which crystallize in the isometric system. The isometric Agl 
exists only above 146 ; below this temperature the iodide is hexagonal. 
The jexhagonal modification occurs as the mineral iodyrite, which, of 
course, is not regarded as a member of the cerargyrite group. 



138 DESCRIPTIVE MINERALOGY 

Cerargyrite (AgCl) 

Cerargyrite, or horn silver, is an important silver ore. It is usually 
associated with other silver compounds, the mixture being mined and 
smelted without separation of the components. It is usually recog- 
nizable by its waxy, massive character. 

Silver chloride consists of 24.7 per cent chlorine and 75.3 per cent 
silver, but cerargyrite often contains, in addition to its essential con- 
stituents, some mercury, bromine and occasionally some iodine. Crystals 
are rare. They are isometric (hexoctahedral class), with a cubical habit, 
their predominant forms being 00 O 00 (100), 00 O(no), O(in), 20(221) 
and 202(211). Twins sometimes occur with O(in) the twinning face. 
The mineral is sometimes found massive, embedded among other min- 
erals, but is more frequently in crusts covering other substances. 

The fracture of cerargyrite is conchoidal. The mineral is sectile. 
Its hardness is 1-1.5 an ^ density about 5.5. Its color is grayish, white 
or yellow, sometimes colorless. On exposure to light it turns violet- 
brown. It is transparent to translucent and its streak is white. It is a 
very poor conductor of electricity. Like halite it is diathermous. n for 
sodium light = 2.071. 

In the closed tube cerargyrite fuses without decomposition. On 
charcoal it yields a metallic globule of silver, and when heated with oxide 
of copper in the blowpipe flame it gives the chlorine reaction. The min- 
eral is insoluble in water and in nitric acid but is soluble in ammonia, and 
potassium cyanide. When a particle of the mineral is placed on a 
sheet of zinc and moistened with a drop of water, it swells, turns black 
and is finally reduced to metallic silver, which, when rubbed by a knife 
blade, exhibits the white luster of the metal. 

Cerargyrite is easily distinguished from all other minerals, except 
the comparatively rare bromide and iodide, by its physical properties and 
by the metallic globule which it yields on charcoal. 

Syntheses. — Crystals of cerargyrite have been obtained by the rapid 
evaporation of ammoniacal solutions of silver chloride, and by the cooling 
of solutions of the chloride in molten silver iodide. 

Occurrence. — The mineral occurs in the upper (oxidized) portions of 
veins of argentiferous minerals, where it is associated with native silver 
and oxidized products of various kinds. 

Localities. — The most important localities of cerargyrite are in Peru, 
Chile, Honduras and Mexico, where it is associated with native silver. 
It is also found near Leadville, Colo,; near Austin, in the Comstock 
lode, Nev., and at the Poorman Mine, and in other mines in Idaho 



CHLORIDES, FLUORIDES, ETC. 139 

and at several places in Utah. Good crystals occur in the Poorman 

Extraction. — When a silver ore consists essentially of cerargyrite the 
metal may be extracted by amalgamation. Ores containing compara- 
tively small quantities of cerargyrite are smelted. 

Production. — The quantity of cerargyrite mined cannot be safely 
estimated. As has been stated, it is usually wrought with other silver 
ores. 

THE FLUORIDES 

The fluorides are salts of hydrofluoric acid. There are several 
known to occur as minerals, but only two, the fluoride of calcium and 



Pig to.— Group of Fluorite Crystals from Wear dale, Co., Durham, England. (Foots 
Mineral Company.) 

the double fluorides of sodium and aluminium are of sufficient impor- 
tance to merit description here. 

Fluorite (CaF 2 ) 
Fluorite, or fluorspar, is the principal source of fluorine. It is usually 
a transparent mineral that is characterized by its fine color and its hand- 



140 



DESCRIPTIVE MINERALOGY 



some crystals (Fig. 60). Perhaps there is no other mineral known that 
can approach it in the beauty of its crystal groups. The uncrystallized 
fluorite may be massive, granular or fibrous. 

Fluorite is a compound of Ca and F in the proportion of 48.9 per cent 
F and 51.1 per cent Ca. Chlorine is occasionally present in minute 
quantities, and Si02, AI2O3 and Fe2<I>3 are always present. A sample of 
commercially prepared fluorite from Marion, Ky., gave: 



CaF 2 


Si0 2 


Al 2 03+Fe 2 03 


CaC0 3 


MgO 


94.72 


1 .22 


.98 


1.82 


.68 



The crystallization is isometric (hexoctrahedral class), and inter- 
penetration twins are frequent. The principal forms observed are 








^ 




Fig. 61. 



Fig. 62. 



Fig. 61. — Crystal of Fluorite witji °o O «> , 100 (a) and °o O2, 210 (e). 
Fig. 62. — Intcrpenetration Cubes of Fluorite, Twinned about O(iu). 



O(iii), ooOoo(ioo), 0002(210) and 402(421) (Fig. 61), but some crys- 
tals are highly modified, as many as 58 forms having been identified upon 
the species. The twins, with O(in) the twinning plane, are usually 
interpenetration cubes, or cubes modified on the corners by the octa- 
hedrons (Fig. 62). The mineral occurs also in granular, fibrous and 
earthy masses. 

The cleavage of fluorite is perfect parallel toO(in). The mineral 
is brittle, its fracture is uneven or conchoidal, its hardness is 4 and its 
density about 3.2. It melts at 1387 . Its color is some shade of yel- 
low, white, red, green, blue cr purple, its luster vitreous, and its streak 
is white. Many specimens are transparent, some are only translucent. 
Most specimens phosphoresce upon heating. A variety that exhibits a 
green phosphoresence is known as chlorophane. The index of refraction 
for sodium light is 1.43385 at 20 . The mineral is a nonconductor of 
electricity. 

The color of the brightly tinted varieties was formerly thought to be 
due to the presence of minute traces of organic substance since it is lost 



CHLORIDES, FLUORIDES, ETC. 141 

or changed when the mineral is heated, but recent observations of the 
effect of radium emanations upon light-colored specimens indicate a 
deepening of their color by an increase in the depth of the blue tints. 
This suggests that the coloring matter is combined with the CaF2. It 
may be a colloidal substance. 

In the closed tube fluorite decrepitates and phosphoresces. When 
heated on charcoal it fuses, colors the flame yellowish red and yields an 
enamel-like residue which reacts alkaline to litmus paper. Its powder 
treated with sulphuric acid yields hydrofluoric acid gas which etches 
glass. The same effect is produced when the powdered mineral is fused 
with four times its volume of acid potassium sulphate (HKSO4) in a 
glass tube. The walls of the tube near the mixture become etched as 
though acted upon by a sand blast. 

Fluorite is easily distinguished by its cleavage and hardness from 
most other minerals. It is also characterized by the possession of 
fluorine for which it gives clear reactions. 

Syntheses. — Crystals are produced upon the cooling of a molten mix- 
ture of CaF2 and the chlorides of the alkalies, and by heating amorphous 
CaF2 with an alkaline carbonate and a little HC1 in a closed tube at 250°. 
Occurrence, Localities and Origin. — The mineral occurs in beds, in 
veins, often as the gangue of metallic ores and as crystals on the walls 
of cavities in certain rocks. It is the gangue of the lead veins of northern 
England and elsewhere. Handsome crystallized specimens come from 
Cumberland and Derbyshire, England; Kongsberg, Norway; Cornwall, 
Wales, and from the mines of Saxony. In the United States the mineral 
forms veins on Long Island; in Blue Hill Bay, Maine; at Putney, in 
Vermont; at Plymouth, Conn.; at Lockport and Macomb, in New 
York; at Amelia Court House, Va., and abundantly in southeastern 
Illinois and the neighboring portion of Kentucky, where it occurs asso- 
ciated with zinc and lead ores. These last-named localities, the neigh- 
borhood of Mabon Harbor, Nova Scotia, and Thunder Bay, Lake 
Superior, afford excellent crystal groups. In nature fluorite has been 
apparently produced both by .crystallization from solutions and by 
pneumatolytic processes. 

Since fluorite is soluble in alkaline waters, its place in the rocks is often 
occupied by calcite, quartz or other minerals that pseudomorph it. 

Uses. — The mineral is used extensively as a flux in smelting iron and 
other ores, in the manufacture of opalescent glass, and of the enamel 
coating used on cooking utensils, etc. It is also used in the manufacture 
of hydrofluoric acid, which, in turn, is employed in etching glass. The 
brighter colored varieties are employed as material for vases and the 



142 DESCRIPTIVE MINERALOGY 

transparent, colorless kinds are ground into lenses for optical instruments. 
The mineral is also cut into cheap gems, known according to color, as 
false topaz, false amethyst, etc. Except when used for making lenses or 
as a precious stone, fluorite is prepared for shipment by crushing, wash- 
ing and screening. A portion is ground. 

Production. — The fluorite produced in the United States is obtained 
mainly from Illinois and Kentucky, though small quantities are mined 
in Colorado, New Mexico and New Hampshire. The production in 
1912 amounted to 116,545 tons, valued at $769,163. Of this, 114,410 
tons came from Illinois and Kentucky. The imports were 26,176 tons, 
valued at $71,616. 

THE DOUBLE CHLORIDES AND DOUBLE FLUORIDES 

These double salts are apparently molecular compounds, in which 
usually two chlorides or two fluorides combine, as in AlF3+3NaF. 
Moreover, one of the members of the combination of chlorides is nearly 
always either the sodium or the potassium chloride. The law of this 
combination is expressed by Professor Remsen in these words: " The 
number of molecules of potassium or sodium chloride which combine 
with another chloride is limited by the number of chlorine atoms con- 
tained in the other chloride." Thus, if NaCl makes double salts with 
MCI2, in which M represents any bivalent element, only two are possible, 
viz: MCb+NaCl and MCl 2 +2NaCl. With MCI3 three double salts 
with sodium may be formed, etc. These double salts are not regarded 
as true molecular compounds, but they are looked upon as compounds 
in which CI and F are bivalent like oxygen. 

Carnallite (KMgCk 6H 2 0) 

Carnallite may be regarded as a hydrated double chloride of the 
composition MgCfe • KC1 • 6H2O with 14.1 per cent K, 8.7 per cent Mg, 
38.3 per cent CI and 39.0 per cent H2O. It occurs in distinct crys- 
tals but more frequently in massive granular aggregates. 

Its crystallization is orthorhombic (bipyramidal class), but the habit 
of its crystals is usually hexagonal because of the nearly equal develop- 
ment of pyramids and brachy domes. Its axial ratio is .5891 : 1 : 1.3759. 
Crystals are commonly bounded by 00 P(no), P(in), ^P(ii2), $P(ii3), 
00 P 06 (010), 2P 06 (021), P 06 (on), I P 06 (023), oP(ooi), and P 00 (101). 
The angle noAi^o=6i° 2o§'. 

Carnallite is colorless lo milky white, transparent or translucent, 
and has a fatty luster. Many varieties appear red in the hand specimens 



CHLORIDES, FLUORIDES, ETC. 143 

because of the inclusion of numerous small plates of hematite or goethite, 
or yellow because of inclusions of yellow liquids or tiny crystals. The 
mineral has a hardness of 1-3, and a density of 1.60. It possesses no 
cleavage but has a conchoidal fracture. It is not an electrical conductor. 
It is deliquescent and has a bitter taste. Its indices of refraction for 
sodium light are a= 1.467, 0= 1.475, T = I -494« 

Before the blowpipe carnallite fuses easily. In the closed tube it 
becomes turbid and gives off much water, which is frequently accom- 
panied by the odor of chlorine. It melts in its own water of crystalliza- 
tion. When evaporated to dryness and heated by the blowpipe flame 
a white mass results which is strongly alkaline. The mineral dissolves 
in water, forming a solution which reacts for Mg, K and CI. 

Carnallite is easily recognized by its solubility, its bitter taste and the 
reaction for chlorine. 

Synthesis. — The mineral separates in measurable crystals from a solu- 
tion of MgCl 2 and KC1. 

Occurrence and Origin. — Carnallite occurs in beds associated with 
sylvite, halite, kieserite (p. 246), and other salts that have been pre- 
cipitated by the evaporation of sea water or the water of salt lakes. 

Localities. — It is found in large quantity at Stassfuxt, Germany; at 
Kalusz, in Galicia and near Maman, in Persia. 

Uses. — Carnallite is used as a fertilizer and as a source of potash 
salts. 

Cryolite (NasAlF*) 

Cryolite usually occurs as a fine-grained granular white mass in 
which are often embedded crystals of light brown iron carbonate (sider- 
ite). The formula given above demands 54.4 per cent F, 12.8 per cent 
Al and 32.8 per cent Na. Analyses of pure white specimens correspond 
veiy closely to this. 

The mineral is monoclinic (prismatic class), but crystals are exceed- 
ingly rare and when found they have a cubical habit. Their axial ratio 
is a : b : ^=.9662 : 1 : 1.3882. £=89° 49'. The principal forms are 
ooP(no), oP(ooi), Poo(oTo), —Poo (010) and Poo (100), thus re- 
sembling the combination of the cube and octahedron. Twins are com- 
mon, with 00 P(no) the twinning plane. 

The cleavage of cryolite is perfect parallel to oP(coi). Its fractuie 
is uneven. Hardness is 2.5 and density about 3. Its color is snow-white 
inclining to red and brown. Its luster is vitreous or greasy and the 
mineral is translucent to transparent. Because of its low index of 
refraction, massive specimens suggest masses of wet snow. The re- 



144 DESCRIPTIVE MINERALOGY 

fractive index f or sodium light is 1.364. It is a nonconductor of 
electricity. 

Cryolite is very easily fusible, small pieces melting even at the low 
temperature of a candle flame. The mineral is soluble in sulphuric acid 
with the evolution of HF. When fused in the closed tube with KHSO4 
it yields hydrofluoric acid, and when fused on charcoal fluorine is evolved. 
The residue treated with Co(N(>3)2 and heated gives the color reaction 
forAl. 

By the aid of its reactions with sulphuric acid, its fusibility and its 
physical properties cryolite is easily distinguished from fluorite, .which it 
most resembles, and from all other minerals. 

Occurrence, Localities and Origin. — The occurrences of cryolite are 
very few. It has been found in smaM quantities near Miask in the 
Ilmen Mts., Russia; near Pike's Peak, Colo., and in the Yellowstone 
National Park. Its principal occurrence is in a great pegmatitic vein 
cutting granite near Ivigtut, Greenland, whence all the mineral used 
in the arts is obtained. The associates of the cryolite at this place are 
siderite, galena, chalcopyrite, p>rite, fluorite, topaz and a few rare 
minerals. The vein is said to be intrusive into the granite. It is 
believed to be a magmatic concentration. 

Uses. — Cryolite was formerly employed principally in the manufac- 
ture of alum and of salts of sodium. At present it is used as a flux in 
the electrolytic production of aluminium, and is employed in the man- 
ufacture of white porcelain-like glass, and in the process of enameling 
iron. The mineral is quarried in Greenland and imported into the 
United States to the extent of about 2,500 tons annually. Its value is 
about $25 per ton. 

THE OXYCHLORIDES 

The oxychlorides are combinations of hydroxides and chlorides. 

Some of them are " double salts " in the sense in which this word is 

explained above. Atacamite is a combination of the oxychloride 

HO\ 
Cu(OH)Cl with the hydroxide Cu(OH) 2 , or >Cu-Cu(OH) 2 . 

CV 

Atacamite (Cu(OH)ClCu(OH) 2 ) 

Atacamite is especially abundant in South America. The mineral 
is usually found in crystalline, fibrous or granular aggregates of a bright 
green color. 

Analyses of specimens from Australia and from Atacama, Chile, yield: 



Cu 


CuO 


H 2 


Total 


14.67 


56.64 


12.02 


99-77 


14.16 


SS- 70 


14-31 


100.00 



CHLORIDES, FLUORIDES, ETC. 146 

CI 

Australia 16.44 

Atacama, Chile *5«83 

The formula requires 16.6 per cent CI, 14.9 per cent Cu, 55.8 per cent 
CuO and 12.7 per cent H2O. 

The crystallization of atacamite is orthorhombic (bipyramidal class), 
with a: b: ^=.6613 : 1 : .7529. Its crystals are usually slender prisms 
bounded by ooP(no), <»P2(i2o), 00 P 06 (010), Poo (on), oP(ooi) 
and P(ni), or tabular forms flattened in the plane of the macropinacoid 
00 Poo (100). Twins are common, with the twinning plane ooP(no). 
The cleavage of atacamite is perfect parallel to 00 P 06 (010). Its 
fracture is conchoidal. Its hardness is 3-3.5 and density about 3.76. 
Pure atacamite is of some shade of green, varying between bright shades 
and emerald. Its aggregates often contain red or brown streaks or 
grains due to the admixture of copper oxides. It is transparent to trans- 
lucent. The streak of the mineral is apple-green. It is a nonconductor 
of electricity. Its indices of refraction for green light are: 0=1.831, 
0= 1.861,7= 1.880. 

In the closed tube atacamite gives off much water with an acid reac- 
tion, and yields a gray sublimate. In the oxidizing flame it fuses and 
tinges the flame azure blue (reaction for copper chloride). It is easily 
reduced to a globule of copper on charcoal and is easily soluble in acids. 
Atacamite is readily distinguished from garnierite, malachite and 
other green minerals by its solubility in acids without effervescence and 
by the azure blue color it imparts to the flame. 

Synthesis. — Crystals have been produced by heating cuprous oxide 
(CU2O) with a solution of FeCl3, in a closed tube at 250 . 

Occurrence, Localities and Origin. — The mineral is most abundant 
along the west side of the Andes Mountains in Chile and Bolivia. It 
occurs also in South Australia; in India; at Ambriz, on the west coast of 
Africa; in southern Spain; in Cornwall, where it forms stalactite tubes; 
in southern California, and near Jerome, Arizona. It is formed as the 
result of the alteration of other copper compounds, and is found most 
abundantly in the upper portions of copper veins. Atacamite changes 
on exposure to the weather into the carbonate, malachite, and the sili- 
cate, chrysocolla. 

Uses. — The mineral is an important ore of copper, but it is mined 
with other compounds and consequently no records of the quantity 
obtained are available. 



CHAPTER VII 

THE OXIDES 

The oxides (except water) and the hydroxides may be regarded as 
derivatives of water, the hydrogen being replaced wholly or in part 
by a metal. When only part of the hydrogen is replaced an hydroxide 
results; when all of the hydrogen is replaced an oxide results. Thus, 
sodium hydroxide, NaHO, may be looked upon as H2O, in which Na has 
replaced one atom of H, and sodium oxide, Na2<3, as H2O in which both 
hydrogen atoms have been replaced by this element. Ferric oxide and 
ferric hydroxide bear these relations to water: 

H-O— H O 

H 





H-— — H, Fe — — Fe, ferric oxide, H — O — Fe, ferric hydroxide. 

,/ Fe 2 3 H-o/ Fe(OH) 3 




H— O— H O 



The oxides constitute a very important, though not a large, class of 
minerals. Some of them are among the most abundant of all minerals. 
They are separated into the following groups: Monoxides, sesqui- 
oxides, dioxides and higher oxides. 

THE MONOXIDES 

Ice (H 2 0) 

The properties of ice are so well known that they need no special 
description in this place. The mineral is never pure, since it contains, 
in all cases, admixtures of various soluble salts. Its crystallization is 
hexagonal and probably trigonal and hemimorphic (di trigonal pyram- 
idal class). Crystals are often prismatic, as when ice .forms the cover- 
ing of water surfaces, or the bodies known as hailstones. In the form 
of snow the crystals are often stellate, or skeleton crystals, and sometimes 

146 



OXIDES 147 

hollow prisms. The principal forms observed on ice crystals are oP(oooi) 
ooP(ioTo), iPCioTa), P(ioTi) and-jPUo+O (Fig. 63). 

The hardness of ice is about 1.5 and its density .9181. It is trans- 
parent and colorless except in large masses when it appears bluish. Its 
fracture is conchoidal. It possesses no distinct cleavage. Its fusing 



Fig. 63. — Photographs of Snow Crystals, Magnified about 15 Diameters. (After 

BtnUey and Perkins^ 

point is o" and boiling point ico°. It is a poor conductor of electricity. 
Its indices of refraction for sodium light at 8° are: «™ 1.3090, «™ 1-3133. 

COPPER OXIDES 

There are two oxides of copper, the red cuprous oxide (Cu;0) and 
the black cupric oxide (CuO). Both are used as ores, the former being 
much more important a source of the metal than the latter. 

Cuprite (C113O) 

Cuprite occurs in crystals, in granular and earthy aggregates and 
massive. The mineral is usually reddish brown or red and thus is easily 
distinguished from most other minerals. Its composition when pure is 
88.8 per cent Cu and n. 2 per cent O. 

In crystallization the mineral is isometric, in the gyroidal hemihedral 
division of the system (pentagonal icositetrahedral class). Its pre- 



148 DESCRIPTIVE MINERALOGY 

dominant forms are ooO°o(ioo), O(in), ooO(no), 0002(210), 
202(211), 20(221) and 30$(3 2 i)> sometimes lengthened out into 
capillary crystals, producing fibrous varieties (var. chalcotriekite). 

The cleavage of cuprite is fairly distinct parallel to O(in). Its frac- 
ture is uneven or conchoidal. Its hardness is 3.5-4 and density about 6. 
The mineral is in some cases opaque; oftener it is translucent or even 
transparent in very thin pieces. By reflected light its color is red, 
brown and occasionally black. By transmitted light it is crimson. When 
gently heated transparent varieties turn dark and become opaque, but 
they reassume their original appearance upon cooling. Its streak is 
brownish red and has a brilliant luster. When rubbed it becomes yellow 
and finally green. The luster of the mineral varies between earthy and 
almost vitreous. It is a poor conductor of electricity, but its con- 
ductivity increases rapidly with rising temperature. Its refractive index 
for yellow light= 2.705. 

In the blowpipe flame cuprite fuses and colors the mantle of the 
flame green. If moistened with hydrochloric acid before heating the 
flame becomes a brilliant azure blue. On charcoal the mineral first 
fuses and then is reduced to a globule of metallic copper. It dissolves in 
strong hydrochloric acid, forming a solution which, when cooled and 
diluted with cold water, yields a white precipitate of cuprous chloride 

(Cu 2 Cl 2 ). 

Cuprite may easily be distinguished from other minerals possessing 
a red streak by the reaction for copper — such as the production of a 
metal globule on charcoal, and the formation of cuprous chloride in con- 
centrated hydrochloric acid solutions by the addition of water. More- 
over, the mineral is softer than hematite and harder than reaglar, cin- 
nabar and proustite. 

Cuprite suffers alteration very readily. It may be reduced to native 
copper, in which case the copper pseudomorphs the cuprite, or, on ex- 
posure to the air it may be changed into the carbonate, malachite, 
pseudomorphs of which after cuprite are common. 

Syntheses. — Crystals of cuprite have frequently been observed on 
copper utensils and coins that had been buried for long periods of time. 
Crystals have also been obtained by long-continued action of NH3 upon 
a mixture of solutions of the sulphates of iron and copper, and by heating 
a solution of copper sulphate and ammonia with iron wire in a closed tube. 

Occurrence, Origin and Localities. — Cuprite often occurs as well 
defined crystals embedded in certain sedimentary rocks in the upper, 
oxidized portions of copper veins, and in masses in the midst of other 
copper ores, from which it was produced by oxidation processes. It is 



OXIDES 149 

found as crystals in Thuringia, in Tuscany, on the island of Elba, in 
Cornwall, Eng., at Chessy, France; and near Coquimb6, in Chile. 
In Chile, in Peru, and in Bolivia it exists in great masses. 

In the United States it occurs at Cornwall, Lebanon Co., Penn. It 
is also found associated with the native copper on Keweenaw Point, 
Mich.; at the copper mines in St. Genevieve Co., Mo.; at Bisbee and 
at other places in Arizona. The fibrous variety known as chalcolrichite 
is beautifully developed at Morenci in the same State. 

Uses. — Cuprite is mined with other copper compounds as an ore of 
copper. 

Melaconite, or Tenorite (CuO) 

Melaconite, or tenorite, is less common than cuprite. It usually 
occurs in massive forms or in earthy masses. Crystals are rare. Its 
composition is 79.8 per cent Cu and 20.2 per cent O. 

In crystallization melaconite is triclinic with a monoclinic habit. 
Its axial ratio is a : b : c— 1.4902 : 1 : 1.3604 and /9=99° 32'. The 
angles a and 7 are both 90 , but the optical properties of the crystals 
proclaim their triclinic symmetry. 

The mineral possesses an easy cleavage parallel to oP(ooi). Its frac- 
ture is conchoidal and uneven, its hardness 3 to 4 and density about 6. 
When it occurs in thin scales its color is yellowish brown or iron gray. 
When massive or pulverulent it is dull black. Its streak is black, chang- 
ing to green when rubbed. Its refractive index for red light is 2.63. 
It is a nonconductor of electricity. 

The chemical reactions of melaconite are precisely like those of cu- 
prite, with the exception that the mineral is infusible. 

Melaconite is distinguished from the black minerals that contain no 
copper by its reaction for this metal. It is distinguished from covellite 
and other dark-colored sulphides containing copper by its failure to give 
the sulphur reaction. 

Syntheses. — Crystals of melaconite have been found in the flues of 
furnaces in which copper compounds and moist NaCl are being treated. 
They have also been obtained by the decomposition of CuCb by water 
vapor. 

Occurrence, Localities and Origin. — The mineral usually occurs associ- 
ated with other ores of copper, from which it has been formed, in part 
at least, by decomposition. It is mined with these as an ore. Thin 
scales are found on the lava of Vesuvius, where it must have been formed 
by sublimation. Masses occur at the copper mines of Ducktown, Tenn. 



150 DESCRIPTIVE MINERALOGY 

Zincite (ZnO) 

Zincite is the only oxide of the zinc group of elements known. It is 
rarely found in crystals. It usually occurs in massive forms associated 
with other zinc compounds. 

Pure zincite is a compound containing 80.3 per cent Zn and 19.7 per 
cent O. Since, however, the mineral is frequently admixed with man- 
ganese compounds it often contains also some manganese and a little 
A iron. A specimen from Sterling Hill, N. J., 

jm\ gave 98.28 per cent ZnO, 6.50 per cent MnO 

//I uV and .44 per cent Fe203. 

//I \\\ Natural crystals of zincite are very rare. 

// pi \\p\ From a study of artificial crystals it is known 
/ / J P \\\ ^ at ^ e m hieral is hexagonal and hemimorphic 
x| j T \ *>| (dihexagonal pyramidal class). The principal 

I J^^ f ^-X ™] forms observed are ooP(ioTo), ooP2(n5o), 

*^-*- ^l w» g. \^ s> oP(oooi), P(ioTi), P2(ii22) and various other 

Fig. 64.— Zincite Crystal pyramids of the 1st and 2d orders. Their habit 

with 00P, 10T0 (m); j s hemimorphic with P(ion) and oP(oooi) at 

P, ion (p) and oP, the op p OS i te en( k f a s h ort columnar crystal 

OOOI (c). /T? . , V 

(Fig. 64). 

The cleavage of zincite is perfect parallel to oP(oooi). Its fracture 
is conchoidal, its hardness 4-4.5 an d density about 5.8. Although color- 
less varieties are known, the mineral is nearly always deep red or orange- 
yellow, due most probably to the manganese present in it. The streak 
of the red varieties is orange- yellow. Its indices of refraction are 
about 2. The mineral is a conductor of electricity. 

When heated in the closed tube the common variety of zincite 
blackens, but it resumes its original color on cooling. With the borax 
bead it gives the manganese reaction. Heated on charcoal it coats the 
coal with a white film, which, when moistened with cobalt solution and 
heated again with the oxidizing flame of the blowpipe, turns green. The 
mineral dissolves in acids. 

When exposed to the atmosphere zincite undergoes slow decomposi- 
tion to zinc carbonate. 

Syntheses. — Zinc oxide crystals are frequent products of the roasting 
of zinc ores in ovens. They have also been produced by the action of 
zinc chloride vapor upon lime and by the action of water upon zinc 
chloride at a red heat. 

Occurrence and Localities. — The mineral occurs only in a few places. 
It is found with other zinc and manganese minerals near Ogdensburg, 



OXIDES 151 

and at Franklin Furnace, in Sussex Co., N. J., in the form of great 
layers in marble, that are bent into troughs. The layers are probably 
veins that were filled from below by emanations from a great underground 
reservoir of igneous rock. 

Uses. — Most of the zincite produced in the United States is used in 
the manufacture of zinc oxide. The ore, which consists of a mixture of 
zincite, franklinite (see p. 199), and willemite (see p. 306), is crushed 
and separated into its component parts by mechanical processes. The 
separated zincite is then mixed with coal and roasted. The zinc oxide 
is volatilized and is caught in tubes composed of bagging. The willemite 
and franklinite are smelted to metallic zinc and the residues are used in 
the manufacture of spiegeleisen. 

Production. — Formerly this mineral, together with the silicate found 
associated with it in New Jersey, constituted the most important source 
of zinc in this country. At present most of the metal is obtained from 
sphalerite. Of the 380,000 tons of zinc in spelter and zinc compounds 
produced in the United States during 191 2 about 69,760 tons were 
made from zincite and the ores associated with it. This had an esti- 
mated value of $9,626,991. 

THE SESQUIOXIDES 

The sesquioxides (R2O3) include a few compounds of the nonmetals 
that are comparatively rare and a group of metallic compounds that 
includes two minerals of great economic importance. One of these, 
hematite (Fe203), is the most valuable of the iron ores. 

ARSENOLITE— CLAUDETITE GROUP 

The only group of the nonmetallic sesquioxides that need be referred 
to in this place comprises those of arsenic and antimony. This is an 
isodimorphous group including four minerals. 



Isometric 




Monoclinic 


Arsenolite 


AS2O3 


Claudetite 


Senarmontite 


Sb 2 03 


Valentinite 



All the minerals of the group are comparatively rare. The isometric 
forms occur in. well developed octahedrons and in crusts covering other 
minerals. They are also found in earthy masses. It is probable that at 
high temperatures the isometric forms pass over into the monoclinic 
modifications, as some of the latter have been observed to consist of 
aggregates of tiny octahedrons. Crystals of claudetite are distinctly 



152 DESCRIPTIVE MINERALOGY 

monoclinic, but they are so twinned as to possess an orthorhombic 
habit. Valentinite crystals, on the contrary, appear to be plainly 
orthorhombic, but their apparent orthorhombic symmetry may be 
due to submicroscopic twinning of the same character as that in 
claudetite, but which in the latter mineral is macroscopic. 

All four minerals occur as weathered products of compounds contain- 
ing As or Sb. They give the usual blowpipe reactions for As or Sb. 
In the closed tube they melt and sublime. 

Arsenolite (AS2O3) is colorless or white. Its specific gravity is 3.7 
and refractive index for sodium light = 1.755. It usually occurs in octa- 
hedrons; or in combinations of O(iu) and ooO(no), but these when 
viewed in polarized light are often seen to be anisotropic. The mineral is 
found also in aggregates of hair-like crystals with a hardness of 1.2. It is 
soluble in hot water, yielding a solution with a sweetish taste. 

Senarmonite (Sb203) is gray or white. Its density is 5.2 and 
n= 2.087 f° r yellow light. Its octahedral crystals are also often aniso- 
tropic; its hardness =2. It is soluble in hot HC1 but is only very 
slightly soluble in water. When heated it turns yellow; but becomes 
white again upon cooling. 

Claudetite (AS2O3) is monoclinic prismatic, with a : b : c = .4040 : 1 

: .3445 and 0=86° 03'. Its white crystals are usually tabular parallel 

to 00 P 00 (010) and are twinned, with 00 P «> (100) the twinning plane. 

Their cleavage is parallel to 00 Pod (010) and their density is 4.15. 

H=2.5. The mineral is an electrical nonconductor. 

Valentinite (Sb20s) is apparently orthorhombic bipyramidal (pos- 
sibly monoclinic prismatic) with a : b : £=.3914 : 1 : .3367. Its crystals 
are tabular or columnar in habit and are very complex. The mineral is 
found also in radial groups of acicular crystals and in granular and 
dense masses. Its color is white, pink, gray or brown, and streak 
white. Its density is 5.77 and hardness 2.5-3. It * s insoluble in HC1. 
It is a nonconductor of electricity. 

CORUNDUM GROUP 

The sesquioxides of aluminium and iron constitute an isomorphous 
group crystallizing in the rhombohedral division of the hexagonal sys- 
tem (ditrigonal scalenohedral class). Both the aluminium and iron 
compounds, corundum and hematite, are of great economic importance. 



OXIDES 163 

Hematite (Fe20a) 

Hematite is one of the most important minerals, if not the most 
important one, from the economic standpoint, since it is the most val- 
uable of all the iron ores. It is known by its dark color and its red 
powder. It occurs in black, glistening crystals, in yellow, brown or red 
earthy masses, in granular and micaceous aggregates and in botryoidal 
and stalactitic forms. 

Chemically, the mineral is Fe20s corresponding to 30 per cent O and 
70 per cent Fe. In addition to these constituents, hematite often con- 
tains some magnesium and some titanium. By increase in the latter 
element it passes into a mineral which has not been distinguished from 
ilmenite (see p. 462). 

The habit of hematite crystals is nearly always rhombohedral. 





Fig. 6$. — Hematite Crystals with R, 10T1 (r); |p2 f 2243 (*); JR 10T4 (u); 00 P2, 

1 1 20 (a) and oR, 0001 (c). 

Their axial ratio is a: c=i : 1.3658, and the predominant forms are 
R(ioTi), iR(iol4), |P2(2243), the prisms ooP(ioTo)and <x>P2(ii2o) 
and often the basal plane (Fig. 65). In addition, about no other forms 
have been identified. The crystals are often tabular, and sometimes 
are grouped into aggregates resembling rosettes. In many cases the 
terminal faces are rounded. A parting is often observed parallel to 
the basal plane, due to the occunence of the mineral in aggregates in 
which each crystal is tabular. 

Hematite has no well defined cleavage. Its fracture is conchoidal or 
earthy. Its crystals are black, glistening and opaque, except in very 
small splinters. These are red and transparent or translucent. Earthy 
varieties are red. The streak of all varieties is brownish red or cherry- 
red. The hardness of the crystallized hematite is 5.5-6.5 and its density 
about 5.2. It is a good conductor of electricity. Its refractive indices 
are: (0=3.22, €=2.94 for yellow light. 

The mineral is infusible before the blowpipe. In the reducing flame 
on charcoal it becomes magnetic, and when heated with soda it is reduced 
to a magnetic metallic powder. It is soluble in strong hydrochloric acid. 



154 DESCRIPTIVE MINERALOGY 

The crystalline and earthy aggregates of hematite to which distinct 
names have been given are: 

Specular, when the aggregate consists of grains with a glistening, 
metallic luster, like the luster of the crystals. When the grains are thin 
tabular the aggregate is said to be micaceous. 

Columnar or fibrous, when in fibrous masses. The color is usually 
brownish red and the luster dull. The botryoidal, stalactic and various 
imitative forms belong here. Red hematite is a compact red variety in 
which the fibrous structure is not very pronounced. 

Red ocher is a red earthy hematite mixed with more or less clay and 
other impurities. 

Clay ironstone is a hard brownish or reddish variety with a dull luster. 
It is usually a mixture of hematite with sand or clay. 

Oolitic ore is a red variety composed of compacted spherical or nearly 
spherical grains that have a concentric structure. 

Fossil ore differs from oolitic ere mainly in the fact that there are 
present in it small shells and fragments of shells that are now composed 
entirely of hematite. 

Mar tile is a pseudomorph of hematite after magnetite. 

Hematite is distinguished from all other minerals by its red powder 
and its magnetism after roasting. 

Syntheses. — Crystals of hematite are obtained by the action of steam 
on ferric chloride at red heat; by heating ferric hydroxide with water 
containing a trace of NH4F to 250 in a closed tube, and by cooling a 
solution of Fe2<33 in molten borax or halite. 

Occurrence and Origin. — Hematite is found in beds with rocks of 
nearly all ages. It occurs also as a deposit on the bottoms of marshy 
ponds, and in small grains in the rocks around volcanic vents. The 
crystallized variety is often deposited on the sides of clefts in rocks near 
volcanoes and on the sides of certain veins. It is produced by sublima- 
tion, by sedimentation and by metasomatic processes. 

Localities. — Handsome crystals occur on the island of Elba; near 
Limoges in France; in and on the lavas of Vesuvius and Etna; at many 
places in Switzerland, Sweden, etc., and at many in the United States. 

Beds of great economic importance occur in the Gogebic, Menominee 
and Marquette districts in Michigan; in the Mesabe and Vermilion 
districts in Minnesota; in the Pilot Knob and Iron Mountain districts 
in Missouri, and in the southern Appalachians, especially in Alabama. 

Uses. — In addition to its use as an ore the fibrous variety of hematite 
is sometimes cut into balls and cubes to be worn as jewelry. The earthy 
varieties are ground and employed in the manufacture of a dark red 



OXIDES 155 

paint such as is used on freight cars, and the powder of some of the mass- 
ive forms is used as a polishing powder. 

Production. — Most of the iron ore produced in the United States is 
hematite, and by far the greater proportion of it comes from the Lake 
Superior region. The statistics for 191 2 follow: 

Quantity (in Long Tons) of Iron Ore Mined in the Several Lead- 
ing States During 1912 

Hematite Other Iron Ores Total 

Minnesota 34,431,000 34431,000 

Michigan 11,191,000 11,191,000 

Alabama 3,814,000 749,ooo 4,563,000 

New York 106,327 1,110,000 1,216,327 

Wisconsin 860,000 860,000 

Tennessee 246,000 171,000 417,000 

Total in U. S 51,345,782 3,804,365 55,i5o,i47 

The total production in 191 2 was valued at about $104,000,000. 

Corundum (AI2O3) 

Corundum is the hardest mineral known, with the exception of dia- 
mond. In consequence of its great hardness an impure variety is used 
as an abrading agent under the name of emery. It is also one of the 
most valuable of the gem minerals. It occurs as crystals and in granular 
masses. 

The mineral is nearly always a practically pure oxide of aluminium of 
the composition AI2O3, in which there are 52.9 per cent Al and 47.1 per 
cent O. The impure varieties usually contain some iron, mainly as an 
admixture in the form of magnetite. 

The axial ratio of corundum crystals is 1 : 1.36. The forms are 
usually simple pyramids, among which ^2(2243) an ^ 1^2(4483) 
are the most common (Fig. 66), and the prism 00 P2(ii2o). The basal 
plane is also common (Fig. 67). Many crystals consist of a series of 
steep prisms and the basal plane, with a habit that may be described as 
barrel-shaped (Fig. 68). The crystals are often rough with rounded 
edges. The prismatic and pyramidal faces are usually striated hori- 
zontally, and the basal plane by lines radiating from the center. 

All corundum crystals are characterized by a parting parallel to the 
basal plane, and often by a cleavage parallel to the rhombohedron, due 
to the presence of lamellae twinned parallel to R(ioTi). The fracture 
of the mineral is conchoidal or uneven. Its density is about 4 and its 



156 



DESCRIPTIVE MINERALOGY 



hardness 9. The mineral possesses a vitreous to adamantine luster. It 
is transparent or translucent. Its streak is uncolored. Its color varies 
from white, through gray to various shades of red, yellow, or blue. 
The blue varieties are pleochroic in blue and greenish blue shades. The 
mineral is a nonconductor of electricity. Its refractive indices for 
yellow light are: «= 1.7690, *= 1.7598. 

Three varieties of corundum are recognized in the arts: Sapphire, 
corundum and emery. 

Sapphire is the generic name for the finely colored, transparent or 
translucent varieties that are used as gems, watch jewels, meter bearings, 
etc. The sapphires are divided by the jewelers into sapphires, possessing 






Fig. 66. 



Fig. 67. 



Fig. 68. 



Fig. 66. — Corundum Crystal with jP2, 4483 (v). 

Fig. 67. — Corundum Crystal with R, 10T1 (r); 00 P2, 1120 (a), and oR, 0001 (c). 

Fig. 68.— Corundum Crystal. Form a, v and c as in previous figures. Also {P2, 

2243 (») and — 2R, 0221 (j). 



a blue color, rubies, possessing a red shade, Oriental topazes, Oriental 
emeralds and Oriental amethysts having respectively yellow, green and 
purple tints. 

Corundum is the name given to dull colored varieties that are ground 
and used as polishing and cutting materials. 

Emery is an impure granular corundum, or a mixture of corundum 
with magnetite (Fe304) and other dark colored minerals. Emery, like 
corundum, is used, as an abrasive. It is less valuable than corundum 
powder because it contains a large proportion of comparatively soft 
material. 

Powdered corundum when heated for a long time with a few drops of 
cobalt nitrate solution assumes a blue color. The mineral gives no 
definite reaction with the beads It is infusible and insoluble. It is 



OXIDES 157 

most easily recognized by its hardness. The mineral alters to spinel 
(p. 196) and to fibrous and platy aluminous silicates. 

Syntheses. — Corundum crystals have been produced artificially in 
many different ways, but only recently has the manufacture of the gem 
variety been accomplished on a commercial scale. Amorphous AI2C3 
dissolves in melted sodium sulphide and crystallizes from the glowing 
mass at a red heat. By melting AI2Q3 in a mass of some fluoride and 
potassium carbonate containing a little chromium, and using compara- 
tively large quantities of material, violet and blue rubies were obtained 
by Fremy and Verneuil. Rubies are also produced by melting AI2O3 
and a little Cr203 for several minutes at a temperature of 2250 C. in 
an electric oven. 

In recent years reconstructed rubies have become a recognized article 
of commerce. These are crystalline drops of ruby material made by 
melting tiny splinters and crystals of the mineral in an electric arc. 

Alundum is an artificial corundum made by subjecting the aluminium 
hydroxide, bauxite, to an intense heat (5ooo°-6ooo°) in an electric 
furnace. 

Occurrence and Origin. — Corundum usually occupies veins in crys- 
talline rocks or is embedded in basic intrusive rocks and in granular 
limestone. The sapphire varieties are also often found as partially 
rounded crystals in the sands of brook beds. The varieties found in 
igneous rocks are primary crystallizations from the magmas producing 
the rocks. The varieties in limestones are the result of metamorphic 
processes. 

Localities. — Sapphires are obtained mainly from the limestone of 
Upper Burma. They are known also to occur in Afghanistan, in Kash- 
mir and in Ceylon. They are occasionally found in the diamond-bearing 
gravels of New South Wales and in the bed of the Missouri River, near 
Helena, Montana. In the United States sapphire is mined near the 
Judith River in Fergus Co., and in Rock Creek in Granite Co., Mont., 
where it occurs in a dike of the dark igneous rock known as monchiquite, 
and is washed from the placers of three streams in the same State. The 
only southern mines that have produced gem material are at Franklin 
and Culsagee, N. C, and from these not any great quantity of stones of 
gem quality have been taken. 

The largest sapphire crystal ever found was taken, however, from 
one of them. It weighs 312 lb., is blue, but opaque. From one of 
these mines, also, came the finest specimen cf green sapphire (Oriental 
emerald) ever found. 

Corundum in commercial quantities occurs on the coast of Malabar, 



158 DESCRIPTIVE MINERALOGY 

in Siam, near Canton, China, and in southeastern Ontario, Canada. 
Emery is obtained from several of the Grecian Islands, more particularly 
Naxos, and from Asia Minor. It is mined in the United States at Chester, 
Mass., and at Peekskill, N. Y. Crystallized corundum occurs near 
Litchfield, Conn.; at Greenwood, Maine; at Warwick and Amity, N. Y. ; 
at Mineral Hill, Penn.; in Patrick Co., Va.; at Corundum Hill and at 
Laurel Creek, Macon Co., N. C, and at various points in Georgia, at 
ail of which places it has been mined. In all the localities within the 
United States the corundum occurs on the peripheries of masses of 
peridotite (olivine rocks). 

Uses. — Corundum, emery and alundum, after crushing and washing, 
are used as abrasives and in the manufacture of cutting wheels. 

Production. — The amount of sapphire produced in the United States 
in 191 2 was valued at $195,505. Most of it was used for mechanical 
purposes, but 384,000 carats were used as gem material. 

Most of the corundum used in the United States is imported from 
Canada, where it occurs in Haliburton, Renfrew and neighboring coun- 
ties in Ontario, as crystals scattered through the coarse-grained crys- 
talline rocks known as syenite, nepheline syenite and anorthosite. 

Most of the emery is also imported. Only 992 tons with a value 
of $6,652 were mined in 191 2. The imports of corundum and emery 
were valued at $501,725, but the importation of these substances is 
gradually diminishing because of the rapid increase in the amounts 
of alundum and carborundum manufactured. In 191 2 the production 
of alundum reached 13,300.000 lb. valued at $796,000, 

THE DIOXIDES 

THE NONMETALLIC DIOXIDES 

There are but few dioxides of the nonmetals that occur as minerals, 
and only one of these, quartz, is abundant. 

SILICA GROUP 

Silica (Si02) occurs in nature in four or five important modifica- 
tions as follows: 

a Quartz, trigonal-trapezohedral class, below 575 . 

j8 Quartz, hexagonal-trapezohedral class, above 575 ° and below 870 . 

Tridymite, rhombic bipyramidal, pseudohexagonal habit. Hex- 
agonal above 117 . 

Cristobalite, tetragonal system, pseudocubic habit. Isometric above 
140 . 



OXIDES 



159 



Chalcedony is regarded by many mineralogists as a form of quartz, 
but its index of refraction for red light is »= 1.537, which is noticeably 
lower than that of either ray in quartz, which is u= 1.5390, €=1.5480 
for the same color. Its hardness also is a little less than that of quartz. 
Some mineralogists believe that all of these properties may be explained 
on the assumption that the mineral is a mass of fine quartz fibers, perhaps 
mixed with other substances, but those who have investigated it by 
high temperature methods are inclined to regard it as a distinct mineral. 

Quartz (Si0 2 ) 

Quartz vies with calcite for the commanding position among the 
minerals. It is very abundant, and appears under a great variety of 





A 



B 



Fig. 69. 



Fig. 70. 



Fig. 69.— Quartz Crystal Exhibiting Rhombohedral Symmetry. R, ion (r); — R, 

01T1 (2) and °°R, 10T0 (m). "» 

Fig. 70. — Ideal (A) and Distorted (B) Quartz Crystals Bounded by same Forms as 

in Fig. 69. 



forms. Often it occurs in distinct crystals. At other times it appears 
as grains without distinct crystal forms, and again it constitutes great 
massive deposits. 

Pure quartz consists of 46.7 per cent Si and 53.3 per cent O. Mass- 
ive varieties often contain, in addition, some opal (Si(OH)4), and traces 
of iron, calcite (CaCOs), clay, and other impurities. 

The crystallization of quartz is in the trapezohedral tetartohedral 
division of the hexagonal system (trigonal-trapezohedral class), at tem- 
peratures below 575 . When formed above this temperature its sym- 
metry is hexagonal trapezohedral (hemihedral). The former is known as 
a quartz, and the latter as quartz. They readily pass one into the 
other at the stated temperature. The axial ratio is 1 : i.x. The prin- 

- - - 2P2 _ 
cipal forms observed are +R(ioii), — R(oni), 00 R(ioio), (1121), 



160 DESCRIPTIVE MINERALOGY 

6P( 

— -=<5i5i) (Fig- 74) and a series of steep rhombohedrons and trapezo- 

hedrons. Although these may all be tetartohedral since ibe geometrical 




Fig. 71. — Etch Figures on Two Quartz Crystals of the Same Form, Illustrating Dif- 
ferences in Symmetry. A. Right- Hand Crystal. B. Left -Hand Crystal. 
(Afltr Ptnfidd.) 



Fie. 72. — Group of Quartz Crystals with Distorted Rhombohedral Faces. {Foote 
Mineral Company.) 

forms of the first four are not distinguishable from the corresponding 
hemihedral ones, the crystals possess a rhombohedral symmetry (Fig. 
69). The angle 10T1 a ^toi = 85° 46'. 



OXIDES 



161 



. Often the +R and the — R faces are equally developed so that they 
appear to belong to the hexagonal pyramid P (Fig. 70A). Their true 
character, however, is clearly brought out by etching, when figures are 
produced on the +R and the — R that are differently situated with 
respect to the edges of the faces (Fig. 71). On the other hand, on many 
crystals some of the R faces are very much enlarged at the expense of 
the others (Fig. 7a). 

The crystals are commonly prismatic. Often they are so dis- 



V 



Fig. 73. 




Fig. ; 



Fig. 73. — Taperini; Quartz Crystal with Rhombohcdral Symmetry. A Combination 

of f , e, m and Two Steep Rhombohedrons. B. Cross-section near Top. 
Fig. 74.— Quartz Crystals Containing °oR, 10T0 (m); R, 10T1 (r); -R, 01T1 (i); 



on B. 






1 (f); 



torted that it is difficult to detect the position of the c axis (Fig. 
70B). The striations on 00 R{ioTo) are, however, always parallel to 
the edges between R and 00 R. When these are sharply marked the 
position of the vertical axis is easily recognized. Many crystals 
taper sharply toward the ends of the c axis. This tapering is due to 
oscillatory combination of the prism 00 R with rhombohedrons 
(Fig. 73). 

The habits of the crystals vary with the crystallization of the quartz. 
On crystals of the phase the +R and — R faces are equally developed 
and trigonal trapezohedrons are absent. The crystals are hexagonal in 



162 



DESCRIPTIVE MINERALOGY 



habit. Crystals of the a phase usually exhibit marked differences in 

the size and character of the rhombohedral planes; and trigonal trape- 

zohedrons may be present on them. Such crystals are usually trigonal 

in habit and prismatic. 

2P2 _ 

The small (n 21) faces on all types of crystals (Fig. 74) are 

2 

always striated parallel to the edge between this plane and +R. By 

their aid the +R can always be distinguished from the — R. This is a 

matter of some practical importance since plates cut from quartz crystals 

possess the power of rotating a ray of polarized light. The plates cut 






Fig. 75. — Supplementary Twins of Quartz. 

C is a combination of A and B in Fig. 74 twinned about 00 P2(ii2o). This is 
known as the Brazil law. 

D is a combination of two crystals like B twinned about c as the twinning axis. 
One is revolved 6o° with reference to the others, thus causing the r and z faces to 
fall together. Swiss law. E is a twin like D, showing portions of planes belonging 
to each individual. It contains also the form s. 



from some crystals turn the ray to the right; those cut from others turn 

it to the left. Crystals that produce plates of the first kind are known 

as right-handed crystals; those that produce plates of the second kind as 

left-handed crystals. Since this property of quartz plates is employed 

in the construction of optical instruments for use in the detection of 

sugars and certain other substances in solution it is important to be 

able to distinguish those crystals that will yield right-handed plates from 

those that will yield left-handed ones. Observation has shown that 

2P2 _ 

when the (n 21) faces are in the upper right-hand corner of the 00 R 

2 

plane immediately beneath +R the crystal is right-handed. When 

these faces are in the upper left-hand corner of this 00 R plane the crystal 



I nterpenet ration twins of quartz are so common that few crystals 
can be observed that do not exhibit some evidence of twinning (Fig. 75). 
The twinning plane is « R, so that the c axes in the twinned individuals 
are parallel and, indeed, often coincident. The R faces and the 00 R 
faces practically coincide in the twinned parts so that the crystals 
resemble untwinned ones. The twinning is exhibited by dull areas of 
— R on bright areas of +R faces and by breaks in the continuity of the 
striations on 00 R. 

Other twinning laws have also been observed in quartz, but their 
discussion as well as the more complete discussion of the mineral's 
crystallization must be left for larger treatises. In the most common of 
these other laws the individuals are twinned about 
Pa(n«). See Fig. 76. 

The fracture of quartz is conchoidal. Its hard- 
ness is 7 and density 2.65. Its luster is vitreous, or 
sometimes greasy. Pure specimens are transparent 
or colorless, but most varieties are colored by the 
addition of pigments or impurities. When the 
coloring matter is opaque it may be present in 

sufficient quantity to render the mineral also opaque. _ , _ 

7. , , . . . , , Fie. ;6.— Quart). 

The streak is colorless in pure varieties, and of some j w i nne ^ about 
pale shade in colored varieties. The mineral is pyro- Pi(u5j). 
electric and circularly polarizing as described above. 
It is an electric insulator at ordinary temperatures. Its refractive 
indices for yellow light are: w= 1.5443. *= 1-5534- 

Quartz resists most of the chemical agents except the alkalies. It 
dissolves in fused sodium carbonate and in solutions of the caustic 
alkalies. It is also soluble in HF and to a very slight degree in water, 
especially in water containing small quantities of certain salts. When 
heated to 575° the a variety passes into the variety; at 870 both 
varieties pass into tridymite, and at 1470 the tridymite passes over into 
cristobalite. Gradual fusion occurs just below 1470°. 

The varieties of quartz have received many different names depend- 
ing largely upon their color and the uses to which they are put. They 
may be grouped for convenience into crystallized and crystalline vari- 
eties. 

The principal crystallized varieties are: 



164 DESCRIPTIVE MINERALOGY 

Rock crystal, the colorless, transparent variety, that often forms 
distinct crystals. This is the variety that is used in optical instruments. 
It includes the Lake George diamonds, rhinestones and Brazilian peb- 
bles. 

Amethyst, the violet-colored transparent variety. 

Rose quartz, the rose-colored transparent variety. 

Citrine ox false topaz, a yellow and pellucid kind. 

Smoky quartz or Cairngorm stone, a smoky yellow or smoky brown 
variety that is often transparent or translucent, but sometimes almost 
opaque. 

The last four varieties are used as gems, the Cairngorm stone being a 
popular stone for mourning jewelry. 

Milky quartz is the white, translucent or opaque variety such as so 
commonly forms the gangue in mineral veins and the material of " quartz 
veins." 

Sagenite is rock crystal including acicular crystals of rutile (Ti02). 

Aventurine is rock crystal spangled with scales of some micaceous 
mineral. 

The principal crystalline varieties are: 

Chalcedony, a very finely fibrous, transparent or translucent waxy- 
looking quartz that forms mamillary or botryoidal masses. Its color is 
white, gray, blue or some other delicate shade. The water that is always 
present in it is believed to be held between the minute fibers, and not to 
be combined with the silica (see also p. 159). 

Carnelian is the name given to a clear red or brown chalcedony. 

Chrysoprase is an apple-green chalcedony. 

Prase is a dull leek- green variety that is translucent. 

Plasma differs from prase in having a brighter green color and in 
being translucent. 

Heliotrope, or bloodstone, is a plasma dotted with red spots of jasper. 

All of the colored chalcedonies are used as gems or as ornamental 
stones. 

Agate is a chalcedony, or a mixture of quartz and chalcedony, varie- 
gated in color. The commonest agates have the colors arranged in 
bands, but there are others, like " fortification agate " in which the 
colors are irregularly distributed, and still others in which the variation 
in color is due to visible inclusions, as in " moss-agates.' ' The different 
bands in banded agates often differ in porosity. This property is taken 
advantage of to intensify the contrast in their colors. The agate is 
soaked in oil, or in some other substance, and is then treated with chem- 
icals that act upon the material absorbed by it. Those bands which 



OXIDES 165 

have absorbed the greater quantity of this material become darker in 
color than those that have absorbed less. 

Onyx is a very evenly banded agate in which there is a marked con- 
trast in colors. Cameos are onyxes in one band of which figures are cut, 
leaving another band to form a background. 

Sardonyx is an onyx in which some of the bands consist of carnelian. 
It is usually red and white. 

Flint, jasper, hornstone and touchstone are very fine grained crystalline 
aggregates of gray, red or nearly black mixture of opal, chalcedony and 
quartz. They are more properly rocks than minerals. Chert is an im- 
pure flint. 

Sandstone is a rock composed of sand grains, most of which are 
quartz, cemented by clay, calcite or some other substance. When the 
cement is quartz the rock is a quartzite. Oilstones, honestones and some 
whetstones are cryptocrystalline aggregates of quartz, very dense and 
homogeneous, except for tiny rhombohedral cavities that are thought to 
have resulted from the solution of crystals of calcite. They are gener- 
ally believed to be beds of metamorphosed chert. 

Syntheses.— Crystallized quartz has been made in a number of ways 
both from superheated aqueous solutions and from molten magmas. 
Crystals have been produced by the action of water containing am- 
monium fluoride upon powdered glass and upon amorphous Si02, and 
by heating water in a closed glass tube to high temperatures. The 
separation of crystals from molten magmas is facilitated by the addition 
of small quantities of a fluoride or of tungsten compounds. 

Occurrence and Origin. — Quartz occurs as an essential constituent of 
many crystalline rocks such as granite, gneiss, etc., and as the almost sole 
component of certain sandstones. It constitutes the greater portion of 
most sands and the material of many veins. It also occurs as pseudo- 
morphs after shells and other organic bodies embedded in rocks, having 
replaced the original substance of which these bodies were composed. 
It is also one of the decomposition products of many silicates. It may 
thus be primary or secondary in origin. It may result from igneous or 
aqueous processes, or it may be a sublimation product. 

Localities. — Quartz is so widely spread in its distribution that only a 
very few of its most interesting localities can be referred to in this place. 

The finest specimens of rock crystals come from Dauphin6, France; 
Carrara, in Tuscany; the Piedmont district, in Italy; and in the United 
States from Middleville, and Little Falls, N. Y.; the Hot Springs, 
Ark., and from several places in Alexander Co., N. C. Smoky quartz 
is found in good crystals in Scotland; at Paris, Me.; in Alexander 



166 DESCRIPTIVE MINERALOGY 

♦ 

Co., N. C, and in the Pike's Peak region of Colorado. The handsomest 
amethysts come from Ceylon, Persia, Brazil, Nova Scotia and the 
country around Lake Superior. Rose quartz occurs in large quantity 
at Hebron, Paris, Albany and Georgetown, Me. 

Fine agates and carnelians are brought from Arabia, India and Brazil. 
They are abundant in the gravels of Agate Bay and of other bays and 
coves on the north shore of Lake Superior. 

Chalcedony is abundant in the rocks of Iceland and the Faroe Islands, 
in those on the northwest side of Lake Superior, and in the gravels of 
the Columbia, the Mississippi and other western rivers. 

The other valuable varieties of the mineral occur largely in the Far 
East. 

Agatized, or silicified, wood of great beauty exists in enormous quan- 
tity in an old petrified forest near Corrizo, Aiiz. It is also found in 
the Yellowstone Park; near Florissant, Colo., and in other places in the 
Far West. This wood has had all of its organic matter replaced mole- 
cule for molecule by quartz in such a manner that its original structure 
has been perfectly preserved. 

Uses. — Rock crystal is used more or less extensively in the construc- 
tion of optical instruments and in the manufacture of cheap jewelry. 
Smoky quartz, amethyst, onyx, carnelian and heliotrope stones are 
used as gems, and agate, prase, chrysoprase and rose quartz as orna- 
mental stones. 

Milky quartz, ground to coarse powder, is employed in the manu- 
facture of sandpaper. Its most extensive use, however, is in the man- 
ufacture of glass and pottery. Earthenware, porcelain and some other 
varieties of potter's ware are vitrified mixtures of clay and ground 
quartz, technically known as "flint." Ordinary glass is a silicate of 
calcium or lead and the alkalies, sodium or potash. It is made by 
melting together soda, potash, lime or lead oxide and ground quartz or 
quartz sand, and coloring with some metallic salt. A pure quartz glass 
is now being made for chemical uses by melting pure quartz sand. 

Quartz is sometimes used as a flux in smelting operations. In the 
form of sandstone, it is used as a building stone, and in the form of sand 
it is employed in various building operations. Bricks cut from dense 
quartzites (very hard and compact sandstones) are often employed 
for lining furnaces. 

The uses of honestones, oilstones, and whetstones are indicated by 
their names. 

Production. — Many varieties of quartz are produced in the United 
States to serve various uses. Vein quartz is crushed and employed 



OXIDES 167 

in the manufacture of wood filler, paints, pottery, scouring soaps, sand- 
paper and abrasives. It is also used in making ferro-silicon, chemical 
ware, pottery, sand-lime brick, quartz glass, etc. The total quantity 
produced for these purposes in 191 2 was 97,874 tons, valued at 
$191,685. 

The largest quantity of quartz produced is in the form of sand, of 
which 38,600,000 tons were marketed in 1912 at a valuation of $15,300,- 
000. Sandstone, valued at $6,900,000, was quarried for building and 
paving purposes. Oilstones, grindstones, millstones, etc., which are 
made from special varieties of sandstone, were produced to the value of 
$1,220,000. 

Gem quartz obtained in 191 2 was valued at about $22,000. This 
comprised petrified wood, chrysoprase, agate, amethyst, rock crystal, 
smoky quartz, rose quartz, and gold quartz (white quartz containing 
particles of gold). 

THE METALLIC DIOXIDES 

The metallic dioxides include the oxides of tin, titanium, manganese 
and lead. Of these the manganese dioxide may be dimorphous, and the 
titanium dioxide is trimorphous. A dioxide of zirconium is also known, 
badddeyite, but it is extremely rare. The mineral zircon (ZrSiOO is 
often regarded as being isomorphous with cassUerite (Sn02) and rutile 
(Ti02) because of the similarity in the crystallization of the three min- 
erals. The three, therefore, are placed in the same group, in which 
case all must be regarded as salts of metallic acids, thus: Ti02 = TiTiO*, 
Sn02=SnSn04, zircon =ZrSi04. Other authorities regard zircon as an 
isomorphous mixture of Ti02 and Si02. In this book zircon is placed 
with the silicates and the other minerals are considered as oxides. 

The two manganese dioxides are polianUe and pyrolusite. The former 
is tetragonal and the latter orthorhombic. It is possible, however, that 
the crystals of pyrolusite are pseudomorphs and that the substance is a 
mixture of polianite and some hydroxide, as it nearly always contains 
about 2 per cent H2O. 

The three titanium oxides are rutile, which is tetragonal; brookite, 
which is orthorhombic, and analase or octahedrite, which is tetragonal. 
Although rutile and anatase crystallize in the same system, their axial 
ratios are different, as are also their crystal habits and their physical 
properties. A few of these differences are indicated below: 

Rutile a : c=i : .6439; Sp. Gr. =4.283; 00^=2.6158; em— 2.9029. 
Anatase =1:1.7771; Sp. Gr. =3.9 ; a>,,a=2.56i8; €«*= 2.4886. 



168 DESCRIPTIVE MINERALOGY 

Of the three modifications of titanium dioxide, anatase may be 
made at a comparatively low temperature. Brookite requires a higher 
temperature for its production, but rutile is producible at both high 
and low temperatures. Under the conditions of nature both brookite 
and anatase pass readily into rutile. 

Of the seven dioxides discussed, four are members of a single group. 

RUTILE GROUP 

The rutile group consists of four minerals apparently completely 
isomorphous, though no mixed crystals of any two have been discovered. 1 
All crystallize in the tetragonal system (ditetragonal bipyramidal class), 
with the same forms and with closely corresponding axial ratios. The 
names of the members of the group and their axial ratios follow: 

Cassiterile (Sn02) ale = i : .6726 

Rutile (Ti0 2 ) = 1 : .6439 

Polianite (Mn02) =1 • .6647 

PlattnerUe (PbOa) - 1 : .6764 

Cassiterite (S11O2) 

Cassiterite, or tinstone, is the only worked ore of tin. It occurs as 
rolled pebbles of a dark brown color in the beds of streams, as fibrous 
aggregates, and as glistening black crystals associated with other min- 
erals in veins. 

The analyses of cassiterite indicate it to be essentially an oxide of 
tin, or, possibly, a stanyl stannate ((SnO)Sn03), with the composition, 
Sn=78.6 per cent; 0=21.4 per cent. The mineral nearly always con- 
tains some iron oxide and often oxides of tantalum, of zinc or of arsenic. 
The presence of iron and tantalum is so general that most crystals of 
cassiterite may be regarded as isomorphous mixtures of (SnO)(Sn03), 
Fe(SnOs) and Fe(Ta03)2. Thus, a crystal from the Etta Mine in the 
Black Hills, S. D., gave Sn0 2 =9436; FeO=i.62; Ta 2 5 =2.42 and 
Si02=i.oo, indicating a mixture of 5 pts. of Fe(Ta03)2, 18 pts. of 
Fe(Sn03) and 303.5 pts. of (SnO)(Sn03). 

The crystals of cassiterite have an axial ratio of 1 : .6726. They are 
usually short prisms in habit. They often consist of the simple com- 
bination P(ni) and P 00(101) (Fig. 77), or of these forms, together 
with 3PI (321) and various prisms (Fig. 78). Twins are common, the 

1 An isomorphous mixture of the rutile and cassiterite molecules has recently 
been described from Greifenstein, Austria, but its existence has not yet been con- 
firmed. 



OXIDES 169 

twinning plane being P oo (101). When the individuals twinned have 
small prismatic faces the resulting combination is often called a visor 
twin (Fig. 79), because of its supposed resemblance to the visor of a 
helmet. By repetition of the twinning very complex groupings are 
produced. The angle m A * ■' ™ 5&° i*)'- 




Fio. 77. 
Fio. 77.— Cassiterite Crystal with P, 
Fio. 7S. — Cassiterite Crystal with s, t and « P, 1 1 



Fig. 78. 
(1) and P « , 101 («). 

k>W; JP|,3«M. 



(m) 



The cleavage of cassiterite is imperfect parallel to on p 00 (100) and 
P(m). Its fracture is uneven. The color of the massive mineral is 
some dark shade of brown by reflected light, and of the crystals black. 
By transmitted light, the mineral is brown or black. Its luster is very 
brilliant, and its streak is white, gray or brown. The purest specimens 




Fig. 79. — Cassiterite Twinned about P 



Visor Twin. 



are nearly transparent, though the ordinary varieties are opaque. Their 
hardness is about 6.5 and density about 7. The mineral is a noncon- 
ductor of electricity. Its refractive indices for yellow light are: «= 1.9965, 
(=2.0931. 

Three varieties of cassiterite are recognized, distinguished by physical 
characteristics. The ordinary variety known as tinstone is crystallized 



170 DESCRIPTIVE MINERALOGY 

or massive. Wood tin is a botryoidal or reniform variety, concentric in 
structure and composed of radiating fibers. The third variety is stream 
tin. This consists of water-worn pebbles found in the beds of streams 
that flow over cassiterite-bearing rocks. 

Cassiterite is only slightly acted upon by acids. It may be reduced 
to a metallic globule of tin only with difficulty, even when mixed 
with sodium carbonate and heated intensely on charcoal. With 
borax it yields slight reactions for iron, manganese or other impurities. 
When placed in dilute hydrochloric acid with pieces of granulated zinc, 
fragments of cassiterite become covered with a dull gray coating of 
metallic tin which can be burnished by rubbing with a cloth or the hand. 
When rubbed by the hand the odor of tin in contact with flesh is easily 
detected. 

The mineral is most easily distinguished from other compounds that 
resemble it in appearance by its high density and its inertness when 
treated with reagents or before the blowpipe. 

Syntheses. — Crystals of cassiterite have been obtained by passing 
steam and vapor of tin chloride or tin fluoride through red-hot porcelain 
tubes, and by the action of tin chloride vapor upon lime. 

Occurrence and Origin. — Tinstone is found as a primary mineral in 
coarse granite veins with topaz, tourmaline, fluorite, apatite and a great 
number of other minerals. It also occurs impregnating rocks, sometimes 
replacing the minerals of which they originally consisted. In these 
cases it is the product of pneumatolytic processes. In many places it 
constitutes a large proportion of the gravel in the beds of streams. 

Localities and Production. — The crystallized mineral occurs at many 
places in Bohemia and in Saxony, at Limoges in France and sparingly 
in a few places in the United States, especially near El Paso, Texas, 
in Cherokee Co., N. C, in Lincoln Co., S. C, and near Hill City, S. D. 
Massive tinstone and stream tin occur in laige enough quantities to be 
mined in Cornwall, England; on the Malay Peninsula and on the islands 
lying off its extremity; in Tasmania; in New South Wales, Victoria 
and Queensland, Australia; in the gold regions of Bolivia; at Durango 
in Mexico, and at various points in Alaska, at some of which there 
are 400 lb. of cassiterite in a cubic yard of gravel. 

The principal tin ore-producing regions of the world are the Straits 
district, including the Malay Peninsula and the islands of the Malay 
Archipelago; Australia; Cornwall, England; the Dutch East Indies, and 
Bolivia. Of the total output of 122,752 tons of tin produced in 1911, 
61,712 tons were made from the Straits ore, 25,312 tons from the ore 
produced in Bolivia and 16,800 tons from Banka ore. Of the total 



OXIDES 171 

quantity of tin produced about 78 per cent is said to come from stream 
tin and 22 per cent from ore obtained from veins. The quantity 
obtained from ore mined in the United States in 191 1 included 61 tons 
from Alaskan stream tin and two tons from the tinstone mined in the 
Franklin Mountains near El Paso, Texas. Mines have been opened in 
San Bernardino Co., California, and in the Black Hills, South Dakota, 
but they have not proved successful. The mines at El Paso, Texas, are 
not yet fully developed, although they promise to be profitable in the 
near future. The crystals are scattered through quartz veins and 
through a pink granite near the contacts with the veins. The average 
composition of the ore is 2 per cent. This is concentrated to a 60 per 
cent ore before being smelted. The production during 1912 was 130 
tons of stream tin from Buck Creek, Alaska. This was valued at 
$124,800. In the following year 3 tons of cassiterite were shipped from 
Gaffney, S. C. The imports of tin into the United States during 191 1 
were 53,527 tons valued at more than $43,300,000. 

Extraction. — The tin is extracted from the concentrated ore by the 
simple process of reduction. Alternate layers of the ore and charcoal 
are heated together in a furnace, when the metal results. This collects 
in the bottom of the furnace and is ladled or run out. The crude metal is 
refined by remelting in special refining furnaces. 

Uses of the Metal, — The metal tin is employed principally for coating 
other metals, either to prevent rusting or to prevent the action upon 
them of chemical reagents. Tmplate is thin sheet iron covered with 
tin. Copper for culinary purposes is also often covered with this metal. 
It is used also extensively in forming alloys wifj^fnpppi^ antjn)^py j 
bismuth and lead. Among the most important of these alloys are 
bronze, bell metal, babbitt metal, gun metal, britannia, pewter and soft 
solder. Its alloy, or amalgam,, with mercury is used in coating mirrors. 
Several of its compounds also find uses in the arts. Tin oxide is an im- 
portant constituent of certain enamels. The chlorides are used exten- 
sively in dyeing calicoes, and the bisulphide constitutes " bronze 
powder " or " mosaic gold," a powder employed for bronzing plaster, 
wood and metals. 

Ruffle (Ti0 2 ) 

Rutile is one of the oxides of the comparatively rare element titanium. 
It occurs commonly in dark brown opaque cleavable masses and in bril- 
liant black crystals. 

Pure rutile consists of 40 per cent O and 60 per cent Ti. Nearly all 
specimens, however, contain in addition some iron, occasionally as much 



172 



DESCRIPTIVE MINERALOGY 



as 9 per cent or 10 per cent, which is probably due to the admixture of 
Fe 2 03 and FeTiOa in solid solution. 

Rutile is peifectly isomorphous with cassiterite. Its axial ratio is 
i : .6439. The principal planes observed on its crystals are practically 
the same as those observed on cassiterite (Fig, 80). Twins are common, 
with Poo (roi) the twinning plane. (Fig. 81.) This twinning is often 
repeated, producing elbow-shaped groups (Fig. 8a), or by further repe- 




Fig. 80.— Rutile Crystals with «*P, ito(m); »P», 100(0); P« 
« P 3 , 310 (0; P 3 , 313 CO wd 3PI. 3» W- 





Fig. 8c— Rutile Eightling Twinned about P °o (101). 
Fig. 81.— Rutile Twinned about P« (101). Elbow Twin. 



tition wheel-shaped aggregates (Fig. 83). In another common law the 
twinning plane is 3P 00 (301) (Fig. 84). The angle 111 ^ili = $6" 52J'. 
The crystals are prismatic and even sometimes acicular in habit. Their 
prismatic planes are vertically striated. 

The cleavage of rutile is quite distinct parallel to 00 P(no) and less 
so parallel to 00 P 00 (100). 

The mineral is reddish brown, yellowish brown, black or bluish 
brown by reflected light and sometimes deep red by transmitted light. 
Many specimens are opaque but some are translucent to transparent. 



OXIDES 



173 



The latter are often pleochroic in tints varying between yellow and 
blood-red. The streak is pale brown. The hardness of the mineral is 
6 to 6.5 and its density about 4.2. It is an electric nonconductor at 
ordinary temperatures. Its refractive indices for yellow light are: 
40=2.6030, €=2.8894. 

Rutile is infusible and insoluble. Its reactions with beads of borax 
and microcosmic salt are usually obscured by the iron present. When 
this metal is present only in small quantities the microcosmic salt bead 
is colorless while hot, but violet when cold, if it has been heated for some 
time in the reducing flame of the blowpipe. 

The most characteristic chemical reaction of rutile is obtained upon 
fusing it with sodium carbonate on charcoal, dissolving the fused mass in 





Fig. 83. Fig. 84. 

Fig. 83.— Rutile Cyclic Sixling Twinned about P «> (101). 

Fig, 84. — Rutile Twinned about 3P <*> (301). Elbow Twin. Forms: °o P2, 210 (A), 

and Poo | 101 (<?). 



an excess of hydrochloric acid and adding to the solution small scraps of 
tin. Upon heating for some little time, the solution assumes a violet 
color. This is a universal test for the metal titanium. 

Some of the dark red and reddish brown massive varieties of rutile 
may be confounded with some varieties of garnet, which, however, are 
much harder. Its density, its infusibility and the reaction for titanium 
serve to characterize the mineral perfectly. 

Pseudomorphs of rutile after hematite and after brookite and ana- 
tase have been described. It often changes into ilmenite and sphene. 

Syntheses. — By the reaction between TiCU and water vapcr in a red- 
hot porcelain tube, crystals of rutile are formed. Twins are produced 
by submitting precipitated titanic acid in a mass of molten sodium tung- 
state to a temperature of 1000 for several weeks. 

Occurrence and Origin. — Rutile is often found as crystals embedded 
in limestone and in the quartz or feldspar of granite and other igneous 



* 



174 DESCRIPTIVE MINERALOGY 

rocks, as long acicular crystals in slates, and as grains in the rock known 
as nelsonite. It occurs also as fine hair-like needles penetrating quartz, 
forming the ornamental stone " fteches d'amour," and as grains in the 
gold-bearing sand regions. When primary it is probably always a 
product of magmatic processes, either crystallizing from a molten magma 
or being the result of pneumatolysis. 

Localities. — Handsome crystals of the mineral occur at Arendal, in 
Norway; in Tyrol, and at St. Gothard and in the Binnenthal, Switzer- 
land. In the United States large crystals have been obtained at Barre, 
Mass.; at Sudbury, Chester Co., Penn.; at Stony Point, Alexander Co., 
N. C; at Graves Mt., in Georgia; at Magnet Cove, in Arkansas, and 
in Nelson Co., Va. In the latter place it occurs in large quantity as 
crystals disseminated through a coarse granite rock. The rock con- 
taining about 10 per cent of rutile is mined as an ore. It constitutes the 
principal source of the mineral in the United States. A second type 
of occurrence in the same locality is a dike-like rock, nelsonite, composed 
of ilmenite and apatite, in which the ilmenite is in places almost 
completely replaced by rutile. 

Uses. — The mineral is not of great economic importance. It is used 
in small quantity to impart a yellow color to porcelain and to give an 
ivory tint to artificial teeth. It is also used in the manufacture of the 
alloy ferro-titanium which is added to steel to increase its strength. 
Recently the use of titaniferous electrodes in arc lights, and the use of 
titanium for filaments in incandescent lamps have been proposed. Some 
of the salts of titanium are used as dyes and others as mordants. Most 
of the ferro-titanium made in the United States is manufactured from 
titaniferous magnetite. 

Production. — The only rutile mined in the United States during 19 13 
came from Roseland, Nelson Co., Virginia. It amounted to 305 tons of 
concentrates containing about 82 per cent Ti02. At the same time there 
were separated about 250 tons of ilmenite (see p. 462) 

Polianite (MnC>2) is usually in groups of tiny parallel crystals and 
as crusts of crystals enveloping crystals of manganite (MnO • OH). Their 
axial ratio is 1 : .6647. The color of the mineral is iron-gray. Its streak 
is black, its hardness 6-6.5 and density 4.99. It dissolves in HC1 evolv- 
ing chlorine. It is distinguished from pyrdusile by its greater hardness 
and its lack of water. The mineral is extremely rare, being found in 
measurable crystals only at Platten in Bohemia. It occurs in pseudo- 
morphs after manganite at a number of other points in- Europe and at a 
few points elsewhere, but in most cases it has not been clearly distin- 



OXIDES 175 

guished from pyrolusite. The rarity of its crystals is regarded by some 
mineralogists as being due to the fact that in most of its occurrences 
polianite is colloidal (a gel). 

Plattnerite (Pb02) is usually massive, but it occurs in prismatic 
crystals near Mullan in Idaho. Their axial ratio is i : .6764. They are 
usually bounded by 00 P 00 (100), 3P 00 (301), P 00 (101), oP (001) and 
often fP(332). The mineral is found also in crusts. Its color is 
iron-black and its streak chestnut-brown. Its hardness is 5-5.5 and 
its density 8.6. It is brittle and is easily fusible before the blow- 
pipe, giving off oxygen and coloring the flame blue. It yields a lead 
bead. It is difficultly soluble in HNO3, but easily soluble in HC1 with 
evolution of chlorine. Plattnerite is found at Leadhills and at Wanlock- 
head, Scotland, and at the " As You Like " Mine near Mullan, Idaho. 



Pyrolusite (Mn0 2 ) 



Pyrolusite is often the result of the alteration of the hydroxide, man- 
ganite, or of polianite. The few measurable crystals that have been 
studied seem to indicate that their form is pseudomorphic after the 
hydroxide. The change by which manganite may pass over into pyro- 
lusite is represented by the reaction 2MnO(OH)+0=2Mn02+H20. 
Pyrolusite may be, however, only a slightly hydrated form of polianite. 

An analysis of a specimen from Negaunee, Mich., gave: 



MnO 





CaO 


BaO 


Si0 2 


Limonite 


H2O 


Total 


79.46 


17.48 


.18 


■38 


.18 


•31 


I.94 


99*93 



Pyrolusite, as usually found, is in granular or columnar masses, or in 
masses of radiating fibers. It is a soft, black mineral with a hardness of 
only 2 or 2.5 and a density of about 4.8. Its luster is metallic and its 
streak black. It is a fairly good conductor of electricity. 

The reactions of this mineral are practically the same as those of 
polianite and manganite (see p. 191), except that only a small quantity 
of water is obtained from it by heating. Upon strong heating it yields 
oxygen, according to the equation 3Mn02=Mn3+302. 

The manganese minerals are easily distinguished from other minerals 
by the violet color they give to the borax bead and by the green product 
obtained when they are fused with sodium carbonate. Pyrolusite is 
distinguished from manganite by its physical properties, and from poli- 
anite by its softness. 



176 DESCRIPTIVE MINERALOGY 

Localities. — Pyrolusite is worked at Elgersberg, near Iimenau in 
Thuringia; at Vorder Ehrensdorf in Moravia; at Platten in Bohemia; 
at Cartersville, Ga.; at Batesville, Ark., and in the Valley of Virginia. 
A manganiferous silver ore containing considerable quantities of pyro- 
lusite is mined in the Leadville district, Colorado, and large quan- 
tities of manganiferous iron ores are obtained in the Lake Superior 
region. 

Uses. — Pyrolusite, together with the other manganese ores with 
which it is mixed, is the source of nearly all the manganese compounds 
employed in the arts. Some of the ores, moreover, are argentiferous 
and others contain zinc. From these silver and zinc are extracted. The 
most important use of the mineral is in the iron industry. In this indus- 
try, however, much of the manganese employed is obtained from man- 
ganiferous iron ores. The alloys spiegeleisen and ferro-manganese are 
employed very largely in the production of an iron used in casting car 
wheels. It is extremely hard and tough. The manganese minerals are 
also used in glass factories to neutralize the green color imparted to glass 
by the ferruginous impurities in the sands from which the glass is made. 
They are also used in giving black, brown and violet colors to pottery 
and some of their salts are valuable mordants. Pyrolusite, finally, is the 
principal compound by the aid of which chlorine and oxygen are pro- 
duced. 

Production. — The United States in 191 2 produced about 1,664 tons 
of manganese ores, valued at $15,723, and all came from Virginia, South 
Carolina and California. In previous years the ores had been mined 
also in Arkansas, Tennessee and Utah. Moreover, there were imported 
into the country 300,661 tons, valued at $1,769,000. Nearly all of this 
was used in the manufacture of spiegeleisen. The domestic product was 
used in the chemical industries largely in the manufacture of manganese 
brick. Of the manganiferous iron ores about 818,000 tons were produced 
in 191 2. These were utilized mainly as ores of iron, though a large por- 
tion was used as a flux. The product of manganiferous silver ores aggre- 
gated about 48,600 tons, all of which was used as a flux for silver-lead 
ores. Nearly all of this came from Colorado. In addition there were 
imported iron-manganese alloys valued at $3,935,000. 

Anatase and Brookite (Ti0 2 )^ 

As has already been stated, the compound Ti02 is trimorphous, one 
form being orthorhombic and the two others tetragonal. Of the latter, 
one has already been described as rutile. The other is anatase, or octa- 
hedrite. The orthorhombic form is known as brookite. Anatase and 



OXIDES 177 

rutile are separated because of the difference in their axial ratios and in 
the habits of their crystals. Both are ditetragonal bipyramidal, but 
a:c for rutile is i : .6439 and for anatase 1 : 1.7771. Brookite is 
orthorhombic bipyramidal with alb: £=.8416 : 1 : .9444. 

Both anatase and brookite have the same empirical composition, 
which is similar to that of rutile. 

Crystals of anatase are usually sharp pyramidal with the form P(in) 
predominating (Fig. 85), blunt pyramidal with $P(ii3) or |P(ii7) 
predominating (Fig. 86), or tabular parallel to oP(ooi). Twins are 
common in some localities, with P 00 (101) the twinning plane. The 
angle 111 A iTi = 82° 91'. 

The mineral is colorless and transparent, or dark blue, yellow, brown 





Fig. 85. Fig. 86. 

Fig. 85. — Anatase Crystal with P, ui'(^). 
Fig. 86. — Anatase Crystal with JP, 113 (s); P, in (/>); IP, 117 (»); 00 P, no (w); 

00 P 00 f 100 (a) and P 00 , 101 (e). 

or nearly black and almost opaque. Its streak is colorless to light 
yellow. Its cleavage is perfect parallel to P and oP and its fracture 
conchoidal. Its hardness is between 5 and 6 and its density is 3.9. This 
increases to 4.25 upon heating to a red heat, possibly due to its partial 
transformation into rutile. The mineral is insoluble in acids except 
hot concentrated H2SO4. It is a nonconductor of electricity. Its 
indices of refraction for yellow light are: a>= 2.5618, c= 2.4886. 

Brookite crystals are usually tabular parallel to 00 P 66 (100) and 
elongated in the direction of the c axis. Nearly all crystals are 
striated in the vertical zone. Although many forms have been identi- 
fied on them, by far the most common is P2(i22). In some cases this is 
the only pyramidal form present, as in the type known as arkansite 
(Fig. 87). Twins are rare, with ooP2~(2io) the twinning plane. The 
angle in A ili = 64° 17'. 



178 



DESCRIPTIVE MINERALOGY 



Brookite may be opaque, translucent or transparent. Its color 
varies from yellowish brown, through brownish red, to black (arkansite). 
Its streak is brownish yellow. Its cleavage is imperfect parallel to 
oo Poo (101), and its fracture uneven or conchoidal. Its hardness is 
5-6 and density about 4. Upon heating its density increases to that of 
rutile. Its refractive indices for yellow light are: a= 2.5832, 0= 2.5856, 
7= 2.7414. It fuses at about 1560 , and is insoluble in acids. 

The chemical properties of both brookite and anatase are similar 
to those of rutile. They are distinguished from rutile by their physical 
properties and their crystallization. 

Both brookite and anatase alter to rutile. 

Syntheses. — Upon heating TiF4 with water vapor at a temperature 





z 



m 



m 




Fio. 87. — Brookite Crystals with 00 P, no (m); JP, 112 (2) and PT, 122 (<?). The 

combination m and e is characteristic for Arkansite. 



below that of vaporizing cadmium, crystals of anatase are produced. 
If the temperature is raised above the point of vaporization of cadmium 
and kept below that of zinc, crystals of brookite result. 

Occurrence. — Brookite and anatase occur as crystals on the walls of 
clefts in crystalline silicate rocks and in weathered phases of volcanic 
rocks. They are mainly pneumatolytic products, the production of the 
one or the other depending upon the temperature at which the Ti(>2 was 
deposited. 

Localities. — Fine brookite crystals are found at St. Gothard, in 
Switzerland; at Pregrattan, in the Tyrol; near Tremadoc, in Wales; 
at Miask, in Russia, and at Magnet Cove, Arkansas. 

Anatase crystals are less common than those of brookite but they 
occur at many points in Switzerland, especially in the Binnenthal; 
near Bourg d'Oisans, France; at many points in the Urals, Russia; in 
the diamond fields of Brazil, and at the brookite occurrences in Arkansas. 



CHAPTER VIII 
THE HYDROXIDES 

The hydroxides, as has already been explained, may be looked upon 
as derivatives of water, in which only a portion of the hydrogen has been 
replaced. The group includes several minerals of economic importance, 
among which is the fine gem mineral opal. All the hydroxides yield 
water when heated in a glass tube, but they do not yield it as readily as 
do salts containing water of crystallization. 

A few of the hydroxides may act as acids forming salts with metals. 
Diaspore, for instance, is an hydroxide of aluminium AlO-OH, or 

/O— H 

A1C , which appears to be able to form salts; at least, the chemical 

X) 

composition of some of the members of an important group of minerals, 
the spinels, may be explained by regarding them as salts of this acid 
(see p. 195). 

Opal (Si0 2 +Aq) 

The true position of opal in the classification of minerals is somewhat 
doubtful. From the analyses made it appears to be a combination of 
amorphous silica and water, or, perhaps, a mixture of silica in some form 
and a hydroxide of silicon. The percentage of water present is variable. 
In some specimens it is as low as 3 per cent, while in others it is as high 
as 13 per cent. The mineral is not known in crystals. It is probably a 
colloid, in which the water is, in part at least, mechanically held in a gel 
of Si02. It occurs only in massive form, in stalactitic or globular masses 
and in an earthy condition. 

When pure the mineral is colorless and transparent. Usually, how- 
ever, it is colored some shade of yellow, red, green or blue, when it is 
translucent or sometimes even opaque. The red and yellow varieties con- 
tain iron oxides and the green, pros opal, some nickel compound. The 
play of color in gem opal is due to the interference of light rays reflected 
from the sides of thin layers of opal material with different densities 
from that of the main mass of the mineral they traverse. The hardness 
of opal is 5.5-6.5 and its density about 2.1. Its refractive index for 
yellow light, »= 1.4401. It is a nonconductor of electricity. 

179 



180 DESCRIPTIVE MINERALOGY 

The principal varieties of opal are: 

Precious opal, a transparent variety exhibiting a delicate play of 
colors, 

Fire opal, a precious opal in which the colors are quite brilliant 
shades of red and yellow, 

Girasol, a bluish white translucent opal with reddish reflections, 

Common opal, a translucent variety without any distinct play of 
colors, 

Cachalong, an opaque bluish white, porcelain-like variety, 

Hyalite, a transparent, colorless variety, usually in globular or 
botryoidal masses, and 

Siliceous sinter, white, translucent to opaque pulverulent accumula- 
tions and hard crusts, deposited from the waters of geysers and other 
hot springs. 

Tripolite and infusorial earth are pulverulent forms of silica in which 
opal is an important constituent. Tripoli is a light porous siliceous 
rock, supposed to have resulted from the leaching of calcareous material 
from a siliceous limestone. Infusorial earth represents the remains of 
certain aquatic forms of microscopic plants known as diatoms. 

Flint and Chert are mixtures of opal, chalcedony and quartz. 

All varieties of opal are infusible and all become opaque when heated. 
When boiled with caustic alkalies some varieties dissolve easily, while 
others dissolve very slowly. 

Syntheses. — Coatings of material like opal have been noted in glass 
flasks containing hydrofluosilicic acid that had not been opened for 
several years. Opal has also been obtained by the slow cooling of a 
solution of silicic acid in water. 

Occurrence. — The mineral occurs as deposits around hot springs. 
It also forms veins in volcanic rocks and is embedded in certain lime- 
stones and slates, where it is probably the result of the solution of the 
siliceous spicules and shells of low forms of life and subsequent deposi- 
tion. It also results from the solution of the calcite from limestones 
containing finely divided silica. 

It is not an uncommon alteration product of silicates. It seems to 
have been deposited from both cold and hot water. 

Localities. — Precious opal is found near Kashan, in Hungary; at 
Zimapan, Quaretaro, in Mexico; in Honduras; in Queensland and 
New South Wales, Australia, and in the Faroe Islands. Common opal is 
abundant at most of these localities and is found also in Moravia, 
Bohemia, Iceland, Scotland and the Hebrides. Hyalite occurs in small 
quantity at several places in New York, New Jersey, North Carolina, 



HYDROXIDES 181 

Georgia and Florida, and common opal, at Cornwall, Penn., and in 
Calaveras Co., California. Common opal and varieties exhibiting a little 
fire have recently been explored in Humboldt and Lander Counties, 
Nevada. Siliceous sinter is deposited at the Steamboat Springs in 
Nevada and geyserite (a globular form of the sinter) at the mouths of the 
geysers in the Yellowstone National Park. 

Uses. — The precious and fire opals are popular and handsome gems. 
Opalized wood, i.e., wood that has been changed into opal in such a 
manner as to retain its woody structure, is often cut and polished for use 
as an ornamental stone. Infusorial earth, a white earthy deposit of 
microscopic shells consisting largely of opal material, possesses many 
uses. It is employed in the manufacture of soluble glass, polishing 
powders, cements, etc., and as the " body," which, saturated with nitro- 
glycerine, composes dynamite. Tripoli, a mixture of quartz and opal, 
is used as a wood filler, in making paint, as an abrasive and in the 
manufacture of filter stones. The principal sources of commercially 
valuable opal material in the United States are the opalized forest in 
Apache Co., Ariz., the infusorial earth beds at Pope's Creek and Dun- 
kirk, Md., various places in Napa Co., Cal., at Virginia City, Nev., 
and at Drakesville, N. J., and the tripoli beds in the neighborhood of 
Stella, Mo., and the adjoining portion of Illinois. 

Production. — The total quantity of infusorial earth and tripoli mined 
during 191 2 was valued at $125,446. The aggregate value of precious 
opal obtained in 191 2 was $10,925. This came from California and 
Arizona. 

Brucite (Mg(OH) 2 ) 

Brucite is the hydroxide of magnesium. It is a white, soft mineral 
usually occurring in crystals or in foliated masses. 

Analyses of the mineral correspond very closely to the formula 
Mg(OH)2 which requires 41.38 per cent Mg, 27.62 per cent O and 31.00 
per cent H2O, though they usually show the presence of small quantities 
of iron and manganese. A specimen from Reading, Perm., yielded: 



MgO 


Fe 2 3 


MnO 


H 2 


Total 


67.64 


.82 


■63 


30.92 


100.01 



The crystallization of brucite is hexagonal (ditrigonal scalenohedral), 
a : c=i : 1.5208. The crystals are tabular in habit in consequence of 
the broad development of the basal plane oP(oooi). The other forms 
present are R(ioTi), — 4R(o44i) and — £R(oil3) (Fig. 88). The angle 
ioTiai"ioi = 97°38'. 




182 DESCRIPTIVE MINERALOGY 

The cleavage of brucite is very perfect parallel to oP(ooi), and folia 
that may be split off are flexible. The mineral is sectile. Its hardness 
is 2.5 and its density 2.4. Its color is white, inclining to bluish and 
greenish tints, and its luster pearly on oP. Brucite is transparent to 

translucent. It is pyroelectric and a non- 
conductor of electricity. Its refractive indices 
for red light are: (0=1.559, €=1.579. 

In the closed tube brucite, like other hy- 
droxides, yields water. The mineral is infusi- 
ble. When intensely heated, it glows. After 
Fig. 88. — Brucite Crystal heating, it reacts alkaline. When moistened 
with oR, 0001 W; R ^^ cobalt nitIate ao i ution and heated, it turns 

a^- 4 R, I* it)!' P^ the characteristic reaction for magnesium. 

The pure mineral is soluble in acids. 

Brucite resembles in many respects gypsum, talc, diaspore and some 
micas. It is distinguished from diaspore and mica by its hardness and 
from talc by its solubility in acids. Gypsum is a sulphate, hence the 
test for sulphur will sufficiently characterize it. 

Synthesis. — Crystals have been made by precipitating a solution of 
magnesium chloride with an alcoholic solution of potash, dissolving the 
precipitate by heating with an excess of KOH and allowing to cool. 

Occurrence and Origin. — Brucite is usually associated with other 
magnesium minerals. It is often found in veins cutting the rock known 
as serpentine, where it is probably a weathering product, and is some- 
times found in masses in limestone, especially near its contact with 
igneous rocks. 

Localities. — It occurs crystallized in one of the Shetland Islands; at 
the Tilly Foster Iron Mine, Brewster, N. Y.; at Woods Mine, Texas, 
Penn., and at Fritz Island, near Reading, in the same State. 

Gibbsite (Al(OH)s) 

Gibbsite, or hydrargillite, is utilized to some extent as an ore of alu- 
minium. It occurs as crystals, in granular masses, in stalactites and in 
fibrous, radiating aggregates. 

Its theoretical composition demands 65.41 per cent AI2O3 and 
34.59 per cent H2O. Usually, however, the mineral is mixed with bauxite 
(Al20(OH)4) and in addition contains also small quantities of iron, 
magnesium, silicon and often calcium. 

Crystals are monoclinic with a : b : c= 1.709 : 1 : 1.918 and £=85° 
29!'. Their habit is tabular. Besides the basal plane, oP(ooi), the 



HYDROXIDES 183 

two most prominent forms are ooP* (ioo) and ooP(no). Thus the 
plates have hexagonal outlines. They have a perfect cleavage parallel 
to the base. Twinning is common, with oP(ooi) the twinning plane. 

The mineral has a glassy luster except on the basal plane where its 
luster is pearly. It is transparent or translucent, white, pink, green or 
gray. Its streak is light, its hardness is 2-3 and specific gravity 2.35. 
It is a nonconductor of electricity. Its refractive indices are: a=/3 

= i-5347, 7= 1.5577- 

When heated before the blowpipe the mineral exfoliates, becomes 

white, glows strongly but does not fuse. Upon cooling the heated mass 
is hard enough to scratch glass, The mineral dissolves slowly but com- 
pletely in hot HC1 and in strong H2SO4, and gives a blue color when 
moistened with Co(NOs)2 solution and heated. 

Gibbsite resembles most closely bauxite, from which it is distin- 
guished principally by its structure. It differs from wavellite (p. 287), 
which it also sometimes resembles, in the absence of phosphorus. 

Syntheses. — Crystals of gibbsite have been made by heating on a 
water bath a saturated solution of Al(OH)3 in dilute ammonia until all 
of the ammonia evaporates; and also by gradually precipitating the 
hydroxide from a warm alkaline solution by means of a slow stream 
of C0 2 . * 

Occurrence.^wThe mineral rarely occurs in pure form. It is found in 
veins and in cavities in various schistose and igneous rocks. It is prob- 
ably a weathering product of aluminous silicates. 

Localities. — Gibbsite has been reported as existing in small quantities 
at various points in Europe, near Bombay, India, and at several places 
in South America and Africa. In the United States it occurs at Rich- 
mond, Mass., at Union Vale, Dutchess Co., N. Y., and mixed with 
bauxite at several of the occurrences of this mineral (see page 186). 

Uses, — It is mined with bauxite as a source of aluminium. 

Limonite (Fe 4 3 (OH) tt ) 

Limonite is an earthy or massive reddish brown mineral whose 
composition and crystallization are but imperfectly known. It is an 
important iron ore called in the trade " brown hematite." 

The analyses of limonite. range between wide limits, largely because 
of the great quantities of impurities mixed with it. The formula de- 
mands 59.8 per cent Fe, 25.7 per cent O and 14.5 per cent water, but the 
percentages of these constituents found in different specimens only 
approximately correspond to these figures. Many mineralogists regard 



DESCRIPTIVE MINERALOGY 



Fig. 89.— Limonite Stalactites in Silverbow Mine, Butte, Mont. (After W. H. Weed.} 



Fig. 90. — Botryoidal Limonite. 



HYDROXIDES 185 

limonite as colloidal goethite (FeO-OH) with one molecule or more of 
H2O, depending upon temperature. The principal impurities are clay, 
sand, phosphates, silica, manganese compounds and organic matter. 
The great variety of these is thought to be due to the fact that the 
limonite, like other gels, possesses the power of absorbing compounds 
from their solution, so that the mineral is in reality a mixture of col- 
loidal iron hydroxide and various compounds which differ in different 
occurrences. 

The mineral occurs in stalactites (Fig. 89), in botiyoidal forms (Fig. 
90), in concretionary and clay-like masses and often as pseudomorphs 
after other minerals and after the roots, leaves and stems of trees. 

Limonite is brown on a fresh fracture, though the surface of many, 
specimens is covered with a black coating that is so lustrous as to appear 
varnished. Its streak is yellowish brown. Its hardness is a little over 
5 and its density about 3.7. The mineral is opaque and its luster is dull, 
silky or almost metallic according to the physical conditions of the spec- 
imen. Its index of refraction is about 2.5. It is a nonconductor of 
electricity. 

The varieties recognized are: compact, the stalactitic and other 
fibrous forms; ocherous, the brown or yellow earthy, impure variety; 
bog iron, the porous variety found in marshes, pseudomorphing leaves, 
etc., and brown clay ironstone, the compact, massive or nodular 
form. 

In its chemical properties limonite resembles goethite, from which it 
can be distinguished only with great difficulty except when the latter is 
in crystals. From uncrystallized varieties of goethite it can usually be 
distinguished only by quantitative analysis, although in pure specimens 
the streaks are different. 

Occurrence and Origin. — Limonite is the usual result of the decom- 
position of other iron-bearing minerals. Consequently, it is often found 
in pseudomorphs. In almost all cases where large beds of the ore occur 
the material has been deposited from ferriferous water rich in organic 
substances. One of the commonest types of occurrence is " gossan." 
In the production of this type of ore, those portions of veins carrying 
ferruginous minerals are oxidized under the influence of oxygen-bearing 
waters, forming a layer composed largely of limonite which covers the 
upper portion of the veins and hides the original vein matter. Gossan 
ores derived from chalcopyrite and pyrite are common in all regions in 
which these minerals occur. Another type of limonitic ore comprises 
those found in clays derived from limestones by weathering. In such 
deposits the ore occurs as nodules and in pockets in the clay. Ores of 



186 DESCRIPTIVE MINERALOGY 

this type are common in the valleys within the Appalachian Moun- 
tains. Bog iron ores occur in swamps and lakes into which ferruginous 
solutions drain. The iron may come from pyrite or iron silicates in the 
drainage basins of the lakes or swamps. When carried down it is oxi- 
dized by the air and sinks to the bottom. 

Localities. — The mineral occurs abundantly and in many different 
localities. The most important American occurrences are extensive 
beds at Salisbury and Kent, Conn.; at many points in New Jersey, 
Pennsylvania, Michigan, Tennessee, Alabama, Ohio, Virginia and 
Georgia. 

Uses. — Although containing less iron than hematite, on account of 
its cheapness, and the ease with which it works in the furnace, brown 
hematite is an important ore of this metal. The earthy varieties are 
used as cheap paints. 

Production. — The yield of the United States " brown hematite " 
mines for 191 2 was a little over 1,600,000 tons. Of this amount the 
largest yields were: 

Alabama .• . 749,242 tons 

Virginia 398,833 tons 

Tennessee 171,130 tons 

The quantity of ocher produced in the United States during the same 
year amounted to about 15,269 tons, valued at $149,289. Most of it 
came from Georgia. In addition, 8,020 tons were imported. This 
had a value of $148,300. 

Bauxite (Al 2 0(OH) 4 ) 

Bauxite, or beauxite, like limonite, is probably a colloid. At any 
rate it is unknown in crystals. Until recently it possessed but little 
value. It is now, however, of considerable importance as it is the prin- 
cipal source of the aluminium on the market. 

The mineral is apparently an hydroxide of aluminium with the for- 
mula Al20(OH)4 or A1203-2H20 in which 26.1 per cent is water and 
73.9 per cent alumina (AI2O3), but it may be a colloidal mixture of the 
gibbsite and diaspore (p. 190) molecules, or of various hydroxides, 
since its analyses vary within wide limits. A sample of very pure 
material from Georgia gave on analysis: 



A1203 


Fe203 


S1O2 


Ti0 2 


H 2 


62.46 


.81 


4-72 


23 


31.0* 



HYDROXIDES 187 

Bauxite occurs in concretionary grains (Fig. 91), in earthy, clay-like 
forms and massive, usually in pockets or lenses in clay resulting from the 
weathering of limestones or of syenite. It is white when pure, but as 
usually found is yellow, gray, red or brown in color, is translucent to 
opaque and has a colorless or very light streak. Its density is 2.55 
and its hardness anywhere between 1 and*3. Its luster is dull. It is 
a nonconductor of electricity. 

Before the blowpipe bauxite is infusible. In the closed tube it yields 



Fig. 91.— Pisolitic Bauxite, from near Rock Run, Cherokee Co., Ala. 

water at a high temperature. Its powder when intensely heated with a 
few drops of cobalt nitrate solution turns blue. The mineral is with 
difficulty soluble in hydrochloric acid. 

Occurrence and Origin. — Bauxite in some cases may be a deposit from 
hot alkaline waters, but in Arkansas it is a residual weathering product 
of the igneous rock, syenite. It occurs in beds associated with corundum, 
clay, gibbsite and other aluminium minerals. 

Localities. — Large deposits of the ore occur at Baux, near Aries, 
France; near Lake Wochein, in Carniola; in Nassau; at Antrim, Ire- 
land; in a stretch of country between Jacksonville, Fla., and Carters- 



188 DESCRIPTIVE MINERALOGY 

ville, Ga.; in Saline and Pulaski Counties, Ark.; in Wilkinson Co., Ga., 
and near Chattanooga, Tenn. 

Preparation. — The ore is mined by pick and shovel, crushed and 
washed. It is then, in some cases, dried and broken into fine particles. 
The fine dust is separated from the coarser material, and the latter, 
which comprises most of the ore, is heated to 400 . This changes the 
iron compounds to magnetic oxide which is separated electro-mag- 
netically. The concentrate contains about 86 per cent of AI2O3. This 
is then purified and dissolved in a molten flux, in some cases cryolite, 
and is subjected to electrolysis. The quantity of aluminium made in the 
United States during 191 2 was over 65,600,000 lb., valued at about 
$17,000,000. The value of the aluminium salts produced was about 
$3,000,000. 

Uses. — Bauxite (or more properly the mixture of bauxite and gibbs- 
ite) is practically the only commercial ore of aluminium which, on 
account of its lightness and its freedom from tarnish on exposure, has 
become a very popular metal for use in various directions. It is em- 
ployed in castings where light weight is desired and in the manufacture 
of ornaments and of plates for interior metallic decorations. It is also 
employed in the steel industry, and, in the form of wire, for the trans- 
mission of electricity. The mineral is also used in the manufacture of 
aluminium salts, in making alundum (artificial corundum), and bauxite 
brick for lining furnaces, and in the manufacture of paints and alloys. 

Production. — The bauxite mined in the United States during 191 2 
amounted to about 159,865 tons valued at $768,932, the greater portion 
coming from Arkansas. This is about two-thirds the value of the pro- 
duction of. the entire world. 

Psilomelane 

Psilomelane is probably a mixture of colloidal oxides and hydroxides 
of manganese in various proportions. In most specimens there is a 
notable percentage of BaO or K2O present, and in others small quantities 
of lithium and thallium. The barium and potassium components are 
thought to have been absorbed from their solutions. 

The substance occurs in globular, botryoidal, stalactitic, and massive 
forms exhibiting, in many instances, an obscure fibrous structure. Its 
color is black or brownish black and its streak brownish black and 
glistening. Its hardness is 5.5-6 and specific gravity 4.2. 

Psilomelane is infusible before the blowpipe, in some cases coloring 
the flame green (Ba) and in others violet (K). With fluxes it reacts for 



HYDROXIDES 189 

manganese. In the closed tube it yields water. It is soluble in HC1 
with evolution of chlorine. 

It is distinguished from most other manganese oxides and hydroxides 
by its greater hardness. 

Occurrence. — Psilomelane occurs in veins associated with pyrolusite 
and other manganese compounds, as nodules in clay beds, and as coatings 
on many manganiferous minerals. In all cases it is probably a product 
of weathering. 

Localities. — It is found in large quantity at Elgersburg in Thuringia; 
at Ilfeld, Harz; and at various places in Saxony. In the United States 
it occurs with pyrolusite and other ores of manganese at Brandon, Vt. ; 
in the James River Valley, and the Blue Ridge region of Virginia; in 
northeastern Tennessee; at Cartersville, Georgia; at Batesville, Arkan- 
sas; and in a stretch of country about forty miles southeast of San 
Francisco, California. At many of these points it has been mined as an 
ore of manganese. 

Wad 

Wad is a soft, earthy, black or dark brown aggregate of manganese 
compounds closely related to psilomelane. 

It occurs in globular, botryoidal, stalactitic, flaky and porous 
masses, which, in some cases, are so light that they float on water. It 
also occurs in fairly compact layers and coats the surfaces of cracks, 
often forming branching stains, known as dendrites. 

Wad contains more water than psilomelane, of which it appears 
often to be a decomposition product. More frequently it results from 
the weathering of manganiferous iron carbonate. It is particularly 
abundant in the oxidized portions of veins containing manganese car- 
bonates and silicates. 

Wad is easily distinguished from all other soft black minerals, except 
pyrolusite, by the reaction for manganese, and from all other manganese 
compounds, except pyrolusite, by its softness. From pyrolusite it is 
distinguished by its content of water. 

Localities. — It occurs in most of the localities at which other man- 
ganese compounds are found. 

DIASPORE GROUP 

The diaspore group comprises the hydroxides of aluminium, iron 
and manganese, possessing the general formula R"'0(OH). They are 
regarded as hydroxides in which one of the hydrogens in H2O is replaced 
by the group R'"0, thus: H — O — H, water, AlO — O — H, diaspore. These 



190 



DESCRIPTIVE MINERALOGY 



three compounds from a chemical viewpoint, may be looked upon as the 
acids whose salts comprise the spinel group of minerals, which includes 
among others the three important ore minerals magnetite, chromite and 
franklinite. Of the three members of the diaspore group the manganese 
and iron compounds are valuable ores. All are orthorhombic, in the 
rhombic bipyramidal class. 

Diaspore (AIO(OH)) 

Diaspore is found in colorless or light colored crystals, in foliated 
masses and in stalactitic forms. 

Its composition is theoretically 85 per cent AI2O3 and 15 per cent 



tf^H5v 



m 




v? 



V 



Fig. 92. — Diaspore Crystals. 00 P 06 , 010 (6); *> P3, 130 (z); 00 P, no (m); 00 P2, 
210 (h); P£, on (e), P2, 212 (s); 00 P2, 120 (/); °o P$, 150 (n); |P|, 
232 (tf). 

H2O, though analyses show it to contain, in addition, usually, some iron 
and silicon. A specimen from Pennsylvania yielded: 



AI2O3 
80.95 



H 2 

14.84 



Fe203 
3.12 



Si0 2 Total 
1.53 100.44 



Other specimens approach the theoretical composition very closely. 

In crystallization the mineral is orthorhombic (rhombic bipyramidal 
class), with a : b : ^=.9372 : 1 : .6039. The crystals are usually pris- 
matic, though often tabular parallel to 00 Poo (010). The principal 
planes observed on them are 00 Poo (010), a series of prisms as 
ooP(no), ooP2(2io), °oP3(i3o), the dome Poo (on) and several 
pyramids (Fig. 92). The planes of the prismatic zone are often ver- 
tically striated. The angle iioAi^o = 86° 17'. 

The cleavage of diaspore is very distinct parallel to the brachy- 
pinacoid. Its fracture is conchoidal and the mineral is very brittle. 
Its hardness is about 6.5 and density 3.4. The luster of the mineral is 
vitreous, except on the cleavage surface, where it is pearly. Its color 



HYDROXIDES 191 

varies widely, though the tint is always light and the streak colorless. 
The predominant shades are bluish white, grayish white, yellowish or 
greenish white. The mineral is transparent or translucent. It is a 
nonconductor of electricity. Its refractive indices for yellow light are: 
a=.i702, 18=1.722, 7 = 1.750. 

In the closed tube diaspore decrepitates and gives off water at a high 
temperature. It is infusible and insoluble in acids. When moistened 
with a solution of cobalt nitrate and heated it turns blue, as do all other 
colorless aluminium compounds. 

In appearance, diaspore closely resembles bruciie (Mg(OH)2), from 
which it may be distinguished by its greater hardness and its aluminium 
reaction with cobalt nitrate. 

Synthesis. — Crystal plates of diaspore have been made by heating at 
a temperature of less than 500 , an excess of amorphous AI2O3 in sodium 
hydroxide, enclosed in a steel tube. At a higher temperature corundum 
resulted. 

Occurrence. — Diaspore occurs as crystals implanted on corundum 
and other minerals, and on the walls of rocks in which corundum is 
found. It is probably in most cases a decomposition product of other 
aluminium compounds. 

Localities. — In Ekaterinburg, Russia, it is associated with emery. 
At Schemnitz, Hungary, it occurs in veins. It is found also in the 
Canton of Tessin, in Switzerland, at various places in Asia Minor, and 
on the emery-bearing islands of the Grecian Archipelago. In the 
United States it is associated with corundum, at Newlin, Chester Co., 
Penn., with emery at Chester, Mass., at the Culsagee corundum mine, 
near Franklin, N. C, and at other corundum mines in the same State. 

Manganite (MnO(OH)) 

Manganite usually occurs in groups of black columnar or prismatic 
crystals and in stalactites. 

The formula MnO(OH) requires 27.3 per cent O, 62.4 per cent Mn 
and 10.3 per cent water, or 89.7 per cent MnO and 10.3 per cent water. 
In addition to these constituents, the mineral commonly contains also 
some iron, magnesium, calcium and often traces of other metals. An 
analysis of a specimen from Langban, in Sweden, yielded: 

Mn 2 3 Fe 2 03 MgO CaO H 2 Total 

88.51 .23 1. 51 .62 9.80 100.67 

The orthorhombic crystals of the mineral have an axial ratio a : b : c 
= .8441 : 1 : .5448. The crystals are nearly all columnar with a series 



192 DESCRIPTIVE MINERALOGY 

of prisms, among which are w P4(4io) and °o P(i 10), and the two lateral 
pinacoids ooP 06(010) and 8 P 60 (100) terminated by oP(ooi) or by 
the domes PS (on), P* (101), and pyramids (Figs. 93 and 04). Cru- 
ciform and contact twins, with the twinning plane P 06 (on), are not 
uncommon (Fig. 95). The prismatic surfaces are 
I vertically striated and the crystals are often in 

bundles. The angle 1 10 A tio— 8o° 20'. 

Cleavage is well defined parallel to 00 P 06 (010) 
and less perfectly developed parallel to qoP(iio). 
The fracture is uneven. The luster of the mineral 
is brilliant, almost metallic. Its color is iron-black 
and its streak reddish brown or nearly black. It 
Fig. 93. — Manganite - K usually opaque but in very thin splinters it is 
Crystal "1 th ™ p ' sometimes brown by transmitted light. Its hard- 
and p"m "ioH") ness ** 4 an< * density about 4-3- The mineral is 
a nonconductor of electricity. 
Manganite yields water in the closed tube and colors the borax bead 
amethyst in the oxidizing flame of the blowpipe. In the reducing flame, 
upon long-continued heating, this color disappears. The mineral dis- 
solves in hydrochloric acid with the evolution of chlorine. It is dis- 



FlO. 94- — Group of Prismatic Manganite Crystals from Ilfeld, Harz. 

tinguished from other manganese minerals by its hardness and crystal- 
lization. 

By loss of water manganite passes readily into pyrolusite (MnOs). 
It also readily alters into other manganese compounds. 

Synthesis. — Upon heating for six months a mixture of manganese 
chloride and clcium caarbonate fine crystals like those of manganite 




HYDROXIDES 193 

have been obtained. Their composition, however, was that of haus- 
mannite, indicating that while manganite was produced, it was changed 
to hausmannite during the process. 

Occurrence, Localities and Origin, — Man- 
ganite occurs in veins in old volcanic rocks, 
and also in limestone. It is found at Ilfeld 
in the Harz; at Ilmenau in Thuringia, and 
at Langban in Sweden, in handsome crys- 
tals. In the United States it occurs at the 
Jackson and the Lucie iron mines, Negaunee, 

Mich., and in Douglas Co., Colo. It is ^ ,, . ^ 

, , , A . . . XT Fig. 95. — Manganite Crystal 

also abundant at various places in New Twhmed about p5(oxi)b 

Brunswick and Nova Scotia. In all cases The forms are <»P. no(«); 
it is a residual product of the weathering of ooPXiaoCOandPa^iaQ;). 
manganese compounds. 

Uses. — Manganite is used in the production of manganese compounds. 
As mined it is usually mixed with pyrolusite, this being the most im- 
portant portion of the mixture. 

Goethite (FeO(OH)) 

This mineral, though occasionally found in blackish brown crystals, 
usually occurs in radiated globular and botryoidal masses. Analyses 
of specimens from Maryland, and from Lostwithiel, in Cornwall, gave: 

FC2O3 Mn203 

Maryland 86.32 

Lostwithiel 89-55 • J 6 

The formula FeO(OH) demands 89.9 per cent Fe203 and 10. 1 per cent 
H2O or 62.9 per cent Fe, 27.0 per cent O and 10. 1 per cent H2O. 

Like diaspore and manganite, goethite is orthorhombic, its axial 
ratio being a : b : ^ = .9185 : 1 : .6068. Its crystals are prismatic or 
acicular with the prisms plainly striated vertically. The forms observed 
are commonly 00 P 66 (010), ooP2~(2io),ooP(no),P66 (on) andP(ni). 
The angle no A 1 10=85° 8'. 

The cleavage of goethite is perfect parallel to 00 P 06 (010) and its 
fracture uneven. Its hardness is 5 and density about 4.4. Its color is 
usually yellowish, reddish or blackish brown and its luster almost 
metallic. In thin splinters it is often translucent with a blood-red color 
and a refractive index of about 2.5. Its streak is brownish yellow. It 
is an electric nonconductor. 



H 2 


Si0 2 


Total 


10.80 


2.88 


100.00 


10.07 


.28 


100.06 



194 DESCRIPTIVE MINERALOGY 

The chemical reactions of the mineral are about the same as those of 



hematite, except that it yields water when heated in the closed tube. 
By this reaction it is easily distinguished from the fibrous varieties of 
hematite, as it is also by its streak. 

Synthesis. — Needles of goethite are produced by heating freshly 
precipitated iron hydroxide for a long time at ioo°. 

Occurrence and Localities. — Goethite is usually associated with other 
ores of iron, especially in the upper portion of veins, where it is produced 
by weathering. It is found near Siegen in Nassau; near Bristol and 
Clifton, England, and in large, fine crystals at Lostwithiel and other 
places in Cornwall. 

In the United States it occurs in small quantity at the Jackson and 
the Lucie hematite mines in Negaunee, Mich.; at Salisbury, Conn.; 
at Easton, Penn., and at many other places. 

Uses. — Goethite is used as an ore of iron, but in the trade it is classed 
with limonite as brown hematite. 



CHAPTER DC 
THE ALUMINATES, FERRITES, CHROMITES AND MANGANITES 

Most of these compounds are salts of the comparatively uncommon 
acids HAIO2, HFeCfe and HC1O2, which may be regarded as metaacids 
derived from the corresponding normal acids by the abstraction of water, 
thus: H3AIO3— H20 = HA102. There are only a few minerals belong- 
ing to the group but they are important. One, magnetite, is an ore of 
iron; another, chromite, is the principal ore of chromium and two others 
are utilized as gems. Most of them are included in the mineral group 
known as the spinels. (Compare p. 189.) 

That there is a manganese acid corresponding to the metaacids of Al, 
Fe and Cr is indicated by the fact that in some of the spinels manganese 
replaces some of the ferric iron, as, for example, in franklinite. This 
suggests that this mineral is an isomorphous mixture of a metaferrite 
and a salt of the corresponding manganese acid (HMnCfe). This may 
be regarded as derived from the hydroxide, Mn(OH)3, by abstraction 
of H2O, thus: HsMn03— H20=HMn02. But there are other man- 
ganous acids. Normal manganous acid is Mn(OH)4, or HUMnC^. If 
from this one molecule of water be abstracted, there remains H2Mn03, 
the metamanganous acid. The manganous salt of the normal acid, 
Mn2Mn04, occurs as the mineral, hausmannite, and the corresponding 
salt of the metaacid, MnMnC>3, as the mineral, braunite. 

SPINEL GROUP 

The spinels are a group of isomorphous compounds that may be 
regarded as salts of the acids AIO(OH), MnO(OH), CrO(OH) and 
FeO(OH), in which the hydrogen is replaced by Mg, Fe and Cr. 

Al— O— Ov 

Thus, spinel, Mg- AI2O4 may be regarded as || /Mg; magnetite, 

Al— O— (K 
Fe— O— (X Cr— O— Ov 

Fe304, as || /Fe; cl.romite, FeCr204, as || /Fe; and 

Fe-O— (X Cr— O-CX 

(Fe • Mn)— 0— <X 
frank 1 inite, as | | y(Zn-Mn-Fe). Chemical compounds of 

(Fe Mn)— O— Cr 

195 



196 



DESCRIPTIVE MINERALOGY 



this general type are fairly numerous, but only a few occur as minerals. 

The most important are the three important ores mentioned above; 

spinel is of some value as a gem stone. 

The spinels crystallize in the holohedral divi- 
sion of the isometric system (hexoctahedral class), 
in well defined crystals that are usually combina- 
tions of O(iii) and ooO(no), with the addition on 
some of 00O00 (ioo), 303(311), 202(211), 5OK531), 
etc. Contact twins are so common with O the 
twinning plane, that this type of twinning is often 
referred to as the spinel twinning (Fig. 96). 




Fig. 96. 
Spinel Twin. 



The complete list of the known spinels is as follows: 



Spinel 

Ceylonite (pleonaste) 

Chlorspinel 

Picotite 

Hercynite 

Gahnite 

Dysluite 

Kreittonite 

Magnetite 

Magnesioferrite 

Franklinite 

Jacobsite 

Chromite 



Mg(A10 2 )2 
(Mg.Fe)(A10 2 ) 2 

Mg((Al-Fe)0 2 ) 2 

(Mg-Fe)((Al-Fe-Cr)0 2 ) 2 

Fe(A10 2 ) 2 

Zn(A10 2 ) 2 

(Zn-Fe-Mn)((Al-Fe)0 2 ) 2 

(Zn-Fe-Mg) ((Al-Fe)0 2 ) 2 

Fe(Fe0 2 ) 2 

Mg(Fe0 2 ) 2 

(Fe • Zn.Mn) ((Fe • Mn)0 2 ) 2 

(Mn-Mg)((Fe-Mn)0 2 ) 2 

(Fe-Mg)(Cr-Fe)0 2 ) 2 



Spinel (Mg(A10 2 ) 2 ) 

Ordinary spinel is the magnesian aluminate, which, when pure, con- 
tains 28.3 per cent MgO and 71.7 per cent 
Al 2 03. Usually, however, there are present 
admixtures of the other isomorphs so that 
analyses often indicate Fe, Al and Cr. 

The mineral usually occurs in isolated 
simple crystals, rarely in groups. The forms 
observed on them are O(iu), ooO(no) and 
303(311), and rarely 00 O 00 (100) (Fig. 97). 

The pure magnesium spinel is colorless or FlG *. 97-— Spinel Crystal 

some shade of pink or red, brown or blue, and T? N \ x " w> ' 

v ' ' \d) and 303, 311 (m). 

is usually transparent or translucent, though 

opaque varieties are not rare. Its streak is white. It possesses a glassy 




ALUMINATES, FERRITES, ETC. 197 

luster, and a conchoidal fracture, but no distinct cleavage. Its hard- 
ness is 8 and its density 3.5-3.6. Its refractive indices vary with the 
color: n for yellow light is 1.7150 for red spinel and 1.7201 for the blue 
variety. 

The mineral is infusible before the blowpipe and is unattacked by 
acids. It yields the test for magnesia with cobalt solution. 

Spinel is easily distinguished from most other minerals by its crys- 
tallization and hardness. It is distinguished from pale-colored garnet 
by its blowpipe reactions, especially its infusibility, and its failure to 
respond to the test for Si02. 

The best known varieties are: 

Precious spinel, which is the pure magnesian aluminate. It is trans- 
parent and colorless or some light shade of red, blue or green. The 
bright red variety is known as ruby spinel and is used as a gem. Its 
color is believed to be due to the presence of chromium oxide. It is 
easily distinguished from genuine ruby by the fact that it is not doubly 
refracting and not pleochroic. 

The best ruby spinels come from Ceylon, where they occur loose in 
sand associated with zircon, sapphire, garnet, etc. 

Common spinel differs from precious spinel in that it is translucent. 
It usually contains traces of iron and alumina. 

Both these varieties of spinel occur in metamorphosed limestones 
and crystalline schists. 

Syntheses. — The spinels have been made by heating a mixture of 
AI2O3 and MgO with boracic acid until fusion ensues; and by heating 
Mg(OH)2 with AICI3 in the presence of water vapor. 

Origin. — Spinel has been described as an alteration product of corun- 
dum and garnet. It is also a primary component of igneous rocks and 
a product of metamorphism in rocks rich in magnesium. 

Uses. — Only the transparent ruby spinels have found a use. These 
are employed as gems. 

Ceylonite, or pleonaste, is a spinel in which a portion of the Mg 
has been replaced by Fe, i.e., is an isomorphous mixture of the magne- 
sian and iron aluminates; thus ((Mg-Fe)(A102)2). It is usually black 
or green and translucent, and has a brownish or dark greenish streak 
and a density =3.5-3.6. 

The analysis of a sample separated from an igneous rock in Madison 
Co., Mont., gave, 

AI2O3 FeO MgO Cr 2 3 Fe20 3 MnO CaO SiCfe Total 
62.09 17.56 15.61 2.62 2.10 tr .16 .55 100.69 



198 DESCRIPTIVE MINERALOGY 

Ceylonite occurs in igneous rocks in the Lake Laach region, 
Germany, and in the Piedmont district, Italy and elsewhere; in meta- 
morphosed limestones at Warwick and Amity, N. Y. ; in the limestone 
blocks enclosed in the lava of Vesuvius; and in dolomite metamor- 
phosed by contact action at Monzoni, Tyrol. 

Picotite, or chrome spinel, is a variety intermediate between spinel 
proper and chromite. Its composition may be represented by the 
formula (Mg-Fe)((Al-Fe- 0)02)2. It occurs only in small crystals in 
basic igneous rocks and in a few crystalline schists. Density =4.1. 

Magnetite (Fe(Fe0 2 ) 2 ) 

Magnetite, the ferrous ferrite, the empirical formula of which is 
FesOi, is a heavy, black, magnetic mineral which is utilized as one of 
the ores of iron. 

The pure mineral consists of 72.4 per cent Fe and 27.6 per cent O. 
Most specimens, however, contain also some Mg and many contain small 
quantities of Mn or Ti. A selected sample of magnetite from the Eliza- 
beth Mine, Mt. Hope, New Jersey, analyzed as follows: 

Fe 2 3 FeO Si0 2 
65 . 26 30 . 20 1 . 38 

Magnetite occurs in crystals that are usually octahedrons or dodeca- 
hedrons, or combinations of the two. Other forms are rare. Twins 
are common. The mineral occurs also as sand 
and in granular and structureless masses. When 
the dodecahedron is present, its faces are fre- 
quently striated parallel to the edge between 
ooO(no) and O(in) (Fig. 98). 

Magnetite is black and opaque and its streak Fig. 98. — Magnetite 
is black. It has an uneven or a conchoidal frac- Crystal, with » o 
ture, but no distinct cleavage. Its hardness is ("<>) and O (in), 

5.5-6 and density 4.9-5.2. It is strongly attracted p^rljlef J Edge' 1 10 
by a magnet and in many instances it exhibits aiM i lllm 
polar magnetism. 

The mineral is infusible before the blowpipe. Its powder dissolves 
slowly in HC1, and the solution reacts for ferrous and ferric iron. 

Magnetite is easily recognized by its color, magnetism, and hardness. 

The mineral weathers to limonite and hematite and occasionally to 
the carbonate, siderite. 



Ti0 2 


AI2O3 


MgO 


CaO 


Other 


Total 


1.09 


•55 


.10 


.68 


•73 


99-99 




ALUMINATES, FERRITES, ETC. 199 

Syntheses. — Crystals have been made by cooling iron-bearing silicate 
solutions; treating heated ferric hydroxide with HC1; and by fusing 
iron oxide and borax with a reducing flame. 

Occurrence end Origin. — The mineral occurs as a constituent of 
many igneous rocks and crystalline schists, and in lenses embedded in 
rocks of many kinds. It also constitutes veins cuttinjg these rocks 
and as irregular masses produced by the dehydration and deoxidation 
of hematite and limonite under the influence of metamorphic processes. 
It occurs also as little grains among the decomposition products of 
iron-bearing silicates, such as olivine and hornblende. 

The larger masses are either segregations from igneous magmas or 
deposits from hot solutions and gases emanating from them. 

Localities. — The localities at which magnetite has been found are so 
numerous that only those of the greatest economic importance may be 
mentioned here. In Norway and Sweden great segregated deposits are 
worked as the principal sources of iron in these countries. In the 
United States large lenses occur in the limestones and siliceous crys- 
talline schists in the Adirondacks, New York, and in the schists and 
granitic rocks of the Highlands in New Jersey. Great bodies are mined 
also at Cornwall, and smaller bodies at Cranberry, and in the Far 
West. 

Extraction. — The magnetite is separated from the rock with which it 
occurs by crushing and exposing to the action of an electro-magnet. 

Production. — The total amount of the mineral mined in the United 
States during 191 2 was 2,179,500 tons, of which 1,110,345 tons came 
from New York, 476,153 tons from Pennsylvania, and 364,673 tons 
from New Jersey. 

Franklinite ((Fe-Zn-Mn)((Fe-Mn)0 2 ) 2 ) 

Franklinite resembles magnetite in its general appearance. It is an 
ore of manganese and zinc. 

It differs from magnetite in containing Mn in place of some of the 
ferric iron in this mineral and Mn and Zn in place of some of its ferrous 
iron. Since it is an isomorphous mixture of the iron, zinc and manganese 
salts of the iron and manganese acids of the general formula R'"0(OH), 
its composition varies within wide limits. The franklinite from Mine 
Hill, N. J., contains from 39 per cent to 47 per cent Fe, 10 per cent to 
19 per cent Mn and 6 per cent to 18 per cent Zn. A specimen from 
Franklin Furnace, N. J., contained, 

Fe203 MnO ZnO MgO CaO SiCfe H 2 Total 
66.58 9.96 20.77 .34 .43 .72 .71 99.51 



200 DESCRIPTIVE MINERALOGY 

Its crystals are usually octahedrons, sometimes modified by the do- 
decahedron and occasionally by other forms. The mineral occurs also 
in rounded grains, in granular and in structureless masses. 

It is black and lustrous and has a dark brown streak. Its fracture 
and cleavage are the same as for magnetite. It is only very slightly 
magnetic. It has a hardness of 6 and a density of 5.15. 

The mineral is infusible before the blowpipe. When heated on 
charcoal it becomes magnetic. When fused with Na2CC>3 in the oxidizing 
flame it gives the bluish green bead characteristic of manganese. Its 
fine powder mixed with Na2COa and heated on charcoal yields the white 
coating of zinc oxide which turns green when moistened with Co(N03)2 
solution and again heated. 

Franklinite is distinguished from most minerals by its color and crys- 
tallization and from magnetite and clromite by its brown streak and 
by its reactions for Mn and Zn. It is also characterized by its associa- 
tion with red zincite and green or pink willemite (p. 306). 

Synthesis. — Crystals of franklinite have been made by heating a 
mixture of FeCk, ZnCk and CaO (lime). 

Occurrence and Origin. — Franklinite occurs at only a few places. Its 
most noted localities are Franklin Furnace and Sterling Hill, N. J., where 
it is associated in a white crystalline limestone with zincite, willemite 
and other zinc and manganese compounds. The deposit is supposed 
to have been produced by the replacement of the limestone through the 
action of magmatic waters and vapors. 

Uses. — The mineral is utilized as an ore of manganese and zinc. 
The ore as mined is crushed and separated into parts, one of which 
consists largely of franklinite. This is roasted with coal, when the zinc 
is driven off as zinc oxide. The residue is smelted in a furnace producing 
spiegeleisen, which is an alloy of iron and manganese used in the man- 
ufacture of certain grades of steel. 

Production. — The quantity of this residuum produced in 191 2 was 
104,670 tons, valued at $314,010. 

Chromite (Fe(Cr0 2 )2) 

Chromite, or chrome-iron, is the principal ore of chromium. It 
resembles magnetite and franklinite in appearance. It occurs in iso- 
lated crystals, in granular aggregates, and in structureless masses. 

Chemically, it is a ferrous salt of metachromous acid, of the theoret- 
ical composition CT2O3 = 68 per cent and FeO =32 per cent, but it usually 
contains also small quantities of AI2O3, CaO and MgO. An analysis of 



ALUMINATES, FERRITES, ETC. 201 

a specimen from Chorro Creek, California, after making corrections for 
the presence of some serpentine, yielded: 



Cr 2 03 


AI2O3 


Fe 2 3 


FeO 


MgO 


MnO 


Total 


56.96 


12.32 


381 


12.73 


14.02 


.16 


100.00 



Its crystals are usually simple octahedrons, but they are not as 
common as those of the other spinels. 

Its color is brownish black and its streak brown. It has a conchoidal 
or uneven fracture and no distinct cleavage. It is usually nonmag- 
netic, but some specimens show slight magnetism because of the ad- 
mixture of the isomorphous magnetite molecule. Its hardness is 5.5 
and its density 4.5 to 4.8. 

The mineral is infusible before the blowpipe. It gives the chromium 
reaction with the beads. If its powder is fused with niter and the fusion 
treated with water, a yellow solution of K.2Cr04 results. When fused 
with Na2COa on charcoal it yields a magnetic residue. 

Chromite is easily distinguished from all other minerals but pico- 
tite by its crystallization and its reaction for chromium. It is distin- 
guished from picotite by its inferior hardness and its higher specific 
gravity. 

Synthesis. — Crystals have been made by fusing the proper constit- 
uents with boric acid and after fusion distilling off the boric acid. 

Occurrence and Origin. — Chromite occurs principally in olivine rocks 
and in their alteration product — serpentine. The mineral is found not 
only as crystals embedded in the rock mass, but also as nodules in it 
and as veins traversing it. It is probably in all cases a segregation from 
the magma producing the rock. In a few places the mineral occurs 
in the form of sand on beaches. 

Localities. — It is widely spread through serpentine rocks at many 
places, notably in Brussa, Asia Minor; at Banat and elsewhere in 
Norway; at Solnkive, in Rhodesia; in the northern portion of New 
Caledonia; at various points in Macedonia; in the Urals, Russia; in 
Beluchistan and Mysore, India. 

In the United States the mineral is known at several points in a belt 
of serpentine on the east side of the Appalachian Mountains, and at 
many points in the Rocky Mountains, the Sierra Nevada and the Coast 
Ranges. It has been mined at Bare Hills, Maryland; in Siskiyou, 
Tehama and Shasta Counties, Colorado; in Converse County, Wyoming; 
and in Chester and Delaware Counties, Pennsylvania; and ; in 1914, 
some was washed from chrome sand at Baltimore, Maryland. 



202 DESCRIPTIVE MINERALOGY 

Metallurgy. — The mineral is mined by the usual methods and con- 
centrated, or, if in large fragments, is crushed. It is then fused with 
certain oxidizing chemicals and the soluble chromates are produced. 
Or the ore is reduced with carbon yielding an alloy with iron. The 
metal is produced by reduction of its oxide by metallic aluminium or by 
electrolysis of its salts. 

Uses. — Chromite is the sole source of the metal chromium, which is 
the chrome-iron alloy employed in the manufacture of special grades 
of steel. Chromium salts are used in tanning and as pigments. The 
crude ore, mixed with coal-tar, kaolin, bauxite, or some other ingredient, 
is molded into bricks and burned, after which the bricks are used as 
linings in metallurgical furnaces. These bricks stand rapid changes of 
temperature and are not attacked by molten metals. 

Production. — The annual production of chromite in the world is 
now about 100,000 tons, of which Rhodesia produces about J, New 
Caledonia about J and Russia and Turkey about fc each. The produc- 
tion of the United States in 191 2 was 201 tons, valued at $2,753. All 
came from California. The imports in the same year were 53,929 tons, 
worth $499,818. 



Chrysoberyl (BeAl 2 4 ) 

Chrysoberyl is a beryllium aluminate, the composition of which is 
analogous to that of the spinels. It may be written Be02(A10)2. Al- 
though theoretically it should contain 19.8 per cent BeO and 80.2 per 
cent AI2O3, analyses of nearly all specimens show the presence also of 
iron and magnesium. 

The mineral differs from spinel in crystallizing in the orthorhombic 
system (bipyramidal class). Its axial ratio is .4707 : 1 : .5823. The 
principal forms observed on crystals are: P(in), 00 P 66 ( 1 00) , 
00 P 66 (010), P 06 (on), 00 P2(i2o) and 2P2(i2i) (Fig. 99). The crystals 
are often twins (Fig. 100), trillings or sixlings, with 3Po6(o3i) the 
twinning plane, forming pseudohexagonal groupings (Fig. 101). Sim- 
ple crystals are usually tabular parallel to 00 P 06 (100), which is striated 
vertically. Consequently, in twins this face exhibits striatums arranged 
feather-like. The angle no A iTo=50° 21'. 

The cleavage of chrysoberyl is distinct parallel to Poo (on), and 
indistinct parallel to 00 P 06 (010) and 00 P 65 (100). Its fracture is 
uneven or conchoidal. Its color is some shade of light green or yellow 
by reflected light. It is transparent or translucent and in some cases is 
distinctly red by transmitted light. It is strongly pleochroic in orange, 



ALUMINATES, FERRITES, ETC. 203 

green and red tints. The mineral is brittle, has a hardness of 8.5 and a 
density of about 3.6. Its refractive indices are: « = 1.7470, (3=1.7484, 

T= I-7565- 

Four distinct varieties are recognized: 

Ordinary, pale green, translucent. 

Gem, yellow and transparent. 

Alexandrite, emerald-green in color, but red by transmitted light, 
transparent, usually in twins. Used as a gem. 

Cat's-eye, a greenish variety exhibiting a play of colors (chatoyancy). 

Before the blowpipe the mineral is infusible. It yields the Al reac- 
tion with Co(N03)2, but otherwise is only slightly affected by the flame. 
It is insoluble in acids. 

Chrysoberyl is characterized by its crystallization and great hard- 




Fig. 99. I-'k;. 100. Fie 



Fig. oo.—Chrysolicryl Crystal with « 1*« , 100 («); « I* w ,010 (*); «P!,i2o(s); 

aP», 121 Mi P, in (•) and PS, mi (fl. 

Flu. 100.— Chrysoberyl Twinned about jp 5 (0.11)- 

Fig. 101. — Chrysoberyl Pscudohexagona] Sibling Twinned about 3P w {031). 

ness. It most closely resembles the beryllium silicate, beryl, in appear- 
ance, but is easily distinguished from this by its crystallization. 

Synthesis. — Crystals have been made by fusing BeO and AI2O3 
with boric acid and then distilling off the boric acid. 

Occurrence and Origin. — Chrysoberyl is found principally in granites 
and crystalline schists and as grains in the sands produced by the erosion 
of these rocks. In its original position the mineral is a separation 
from the magma that produced the rocks. 

Localities. — Its best known localities are in Minas Geraes, Brazil; 
near Ekaterinburg, Ural; in the Moume Mrs., Ireland; at Haddam, 
Conn.; at Greenfield, N. Y.; at Orange Summit, N. Hamp.; and at 
Norway and Stoneham, Me. The alexandrite comes from Ceylon, where 
it occurs as pebbles, and from the Urals. 



204 



DESCRIPTIVE MINERALOGY 



Braunite (MnMnOs) occurs massive and in crystals. The latter 
are tetragonal (ditetragonal bipyramidal class), with a : c=i : .9922. 
They are usually simple bipyramids P(ni). Because of the nearly 
equal value of a and c all crystals are isometric in habit. The angle 
iiiAi^i • 70 7'. Twins are common, with P 00(101) the twinning 
plane. Cleavage is perfect parallel to P(ni). 

The mineral is brownish black to steel-gray in color and in streak. 
Its luster is submetallic. Its hardness is 6-6.5 an d density 4.7. It is 
infusible before the blowpipe. With fluxes it gives the usual reactions 
for manganese. It is soluble in HC1 yielding chlorine. 

It occurs in veins with manganese and other ores in Piedmont, Italy, 
and at Pajsberg and various other places in Sweden, where its origin 
is secondary. 

Hausmannite (Mn 2 Mn04) crystallizes like braunite, but a : c— 
1 : 1. 1573 and its crystals are, therefore, distinctly tetragonal in habit. 
They are usually simply P(ni) or combinations of P(in) and JP(ii3), 
though much more complicated crystals are known. The angle 
in A iTi=6o° 1'. Twins and fourlings (Fig. 102) are common, with 





A B 

Fig. 102. — Hausmannite. (A) Simple Crystal, P, in (p) and oP, 001 (c). 

Fiveling Twinned about P °o (101). 



(B) 



P 00(101) the twinning plane. The cleavage is imperfect parallel to 
oP(ooi). The mineral also occurs in granular masses. 

Hausmannite is brownish black. Its streak is chestnut brown. 
Its hardness is 5-5.5 and density 4.8. Its reactions are the same as 
those of braunite. 

Hausmannite occurs as crystals at Ilmenau, Thiiringia; Ilfeld, 
Harz, and as granular masses in dolomite at Nordmark and several 
other points in Sweden. Like braunite it is probably a decomposition 
product of other manganese minerals. 



CHAPTER X 
THE NITRATES AND BORATES 

THE NITRATES 

The nitrates are salts of nitric acid. Only two are of importance 
to us, saltpeter (KNO3) and chile saltpeter (NaNCfe). Both are color- 
less, or white, crystalline bodies, both are soluble in water and both pro- 
duce a cooling taste when applied to the tongue. The potassium com- 
pound is distinguished from the sodium compound by the flame test. 
Both minerals when heated in the closed tube with KHSO4 yield red 
vapors of nitrogen peroxide (NO2). 

Soda Niter (NaN0 3 ) 

Soda niter, or chile saltpeter, is usually in incrustations on mineral 
surfaces or in massive forms. It consists of 63.5 per cent N2O5 and 
36.5 per cent Na20. 

Its crystals are in the ditrigonal scalenohedral class of the hexagonal 
system with an axial ratio of a : e= 1 : .8297. They are usually rhom- 
bohedrous R(ioTi) in some cases modified by oR(oooi). Apparently 
the mineral is completely isomorphous with calcite (CaCOs). 

Its cleavage is perfect parallel to the rhombohedron. Its hardness 
is under 2, its density about 2.27 and its melting point about 31 2 . 
Its luster is vitreous; color white, or brown, gray or yellow. The min- 
eral is transparent. Its refractive indices for yellow light are: to = 1 .5854, 
€=1.3369. 

Soda niter deflagrates when heated on charcoal and colors the flame 
yellow. When exposed to the air it attracts moisture and finally lique- 
fies. It is completely soluble in three times its own weight of water. 

Occurrence and Localities. — The principal occurrences of the mineral 
are in the district of Tarapaca, northern Chile, where, mixed 
with the iodate and other salts of sodium and potassium, under the 
name caliche y it comprises beds several feet thick on the surface of rain- 
less pampas, and in Bolivia at Arane under the same conditions. It is 
associated with gypsum, salt and other soluble minerals. Smaller 

205 



206 DESCRIPTIVE MINERALOGY 

deposits are found in Humboldt Co., Nevada, in San Bernardino Co., 
Cal., and in southern New Mexico. 

The material is thought to result from the action of microorganisms 
upon organic matter decomposing in the presence of abundant air. 

Uses. — Soda niter is used in the production of nitric acid, and in the 
manufacture of fertilizers and gunpowder. About 480,000 tons are 
imported into the United States annually at a cost of $15,430,000. 
Most of it comes from Chile. 

Since soda niter usually contains sodium iodate as an impurity, the 
mineral is an important source of iodine. 

Niter (KNO3) 

Niter, or saltpeter, resembles soda niter in appearance. It gener- 
ally occurs in crusts, in silky tufts and in groups of acicular crystals. 
Its crystals are orthorhombic with a : b : £=.5910 : 1 : .7011. Their 
habit is hexagonal. The principal forms observed on them are 00 P(i 10), 
00 Poo (100), 00 P 66 (010), oP(ooi), P(in), and a series of brachy- 
domes. In many respects the mineral is apparently isomorphous with 
aragonite which is the orthorhombic dimorph of calcite. At 126 it 
passes over into an hexagonal (trigonal) form. Its cleavage is perfect 
parallel to P 66 (on). Its fracture is uneven; its hardness 2 and den- 
sity 2.1. Its medium refractive index for yellow light, /3= 1.5056. 

Niter deflagrates more violently than soda niter and detonates with 
combustible substances. It fuses at about 335 °. It colors the blowpipe 
flame violet. It is soluble in water. 

Occurrence and Localities. — The mineral forms abundantly in dry 
soils in Spain, Egypt, Persia, Ceylon and India, where it is produced 
by a ferment, and on the bottoms of caves in the limestones of Madison 
Co., Ky., of Tennessee, of the valley of Virginia and of the Mississippi 
Valley. 

Production. — Most of the niter used in the arts is manufactured, but 
some is obtained from the deposits in Ceylon and in India. The 
amount imported in 191 2 aggregated 6,976,000 lb., valued at $226,851. 

THE BORATES 

The borates are salts of boric acid, H3BO3, metaboric acid, HBO2, 
tetraboric acid, H2B4O7, hexaboric acid, H4B6O11, and various poly- 
boric acids in which boron is present in still larger proportion. The 
metaacid is obtained from the orthoacid by heating at ioo°, at which 



NITRATES AND BORATES 



207 



temperature the former loses one molecule of water, thus: H3BO3 — 
H20=HB02, and the tetraacid by heating the same compound to 160 
at which temperature 5 molecules of water are lost from 4 molecules of 
the acid, thus: 4H3BO3— 5H2O =1128407. Hexaboric acid may be 
regarded as the orthoacid less 1$ molecules of water, thus: 6H3BO3 
-7H 2 O=H4B Oii. 

Only three of the borates are important enough to be discussed 
here. These are borax, a sodium tetraborate (Na2B407*ioH20), cole- 
manite, a hexaborate (Ca2BeOn-5H20) and boracite, a magnesium 
chloro-polyborate (Mg5(MgCl)2Bi 6 03o). Borax and colemanite are 
commercial substances that are produced in large quantities. 

All borates and many other compounds containing boron when 
pulverized and moistened with H2SO4 impart an intense yellow-green 
color to the flame. If boron compounds are dissolved in hydrochloric 
acid, the solution will turn turmeric paper reddish brown after drying 
at ioo°. The color changes to black when the stain is treated with 
ammonia. 




% 



m 




Borax (Na 2 B 4 7 10H2O) 

Borax occurs as crystals and as a crystalline cement between sand 
grains around salt lakes, as an incrustation on the surfaces of marshes 
and on the sands in desert regions, and dissolved 
in the water of certain lakes in deserts. It 
occurs also as bedded deposits interlayered with 
sedimentary rocks. 

The composition of borax is 16.2 per cent 
Na20, 36.6 per cent B2O3 and 47.2 per cent H2O. 

Crystals are monoclinic (prismatic class), with 
alb: c= 1.0995 : x : 5629, an d £=73° 25'. 
They are prismatic in habit and in general form 
resemble very closely crystals of pyroxene. 
The principal planes occurring on them are 
ooPoo(ioo), ooP(no), oP(ooi), — P(m) and 
— 2P(22i) (Fig. 103). Their cleavage is perfect 
parallel to 00 P 00 (100), and their fracture conchoidal. The angle 

iioaiTo=93°- 

The mineral has a white, grayish or bluish color and a white streak. 

It is brittle, vitreous, resinous or earthy; is translucent or opaque; has 

a hardness of 2-2.5, a density of 1. 69-1. 72, and a sweetish alkaline taste. 

On exposure to the air the mineral loses water and tends to become white 



Fig. 103. — Borax Crystal 
with 00 P, no (m); 
00 P 00 , 100 (a); 00 P 5b , 
010 (b); oP, 001 (c); 
P, In (0) and 2P, 221 

W- 



208 DESCRIPTIVE MINERALOGY 

and opaque, whatever its color in the fresh condition. Its medium 
refractive index for yellow light, 0= 1.4686. 

Before the blowpipe borax puffs up and fuses to a transparent 
globule. Fused with fluorite and potassium bisulphate it colors 
the flame green. It is soluble in water, yielding a weakly alkaline 
solution. 

Occurrence. — The principal method of occurrence of the mineral is 
as a deposit from salt lakes in arid regions, and as incrustations on the 
surfaces of alkaline marshes overlying buried borax deposits. The 
original beds were deposited by the evaporation to dryness of ancient 
salt lakes, and the incrustations were produced by the solution of these 
deposits by ground water, and the rise of the solutions to the surface by 
capillarity. 

Localities. — Borax occurs in the water of salt lakes in Tibet; of 
several small lakes in Lake County, and of Borax Lake in San Bernardino 
County in California, and in the mud and marshes around their borders. 
It occurs also in the sands of Death Valley in the same State, and in 
various marshes in Esmeralda County, Nevada. Other large deposits 
are found in Chile and Peru. 

Uses. — Borax is used as an antiseptic, in medicine, in the arts for 
soldering brass and welding mqf als, and in the manufacture of cosmetics. 

« 

Boric acid obtained from borax and colemanite is employed in the 
manufacture of colored glazes, in making enamels and glass, as an 
antiseptic and a preservative. Some of the borates are used as pig- 
ments. 

Production. — Borax was formerly obtained in the United States, 
especially in California, Oregon and Nevada, by the evaporation of 
the water of borax lakes, by washing the crystals from the mud on their 
bottoms and by the leaching of the mineral from marsh soil. At pres- 
ent, however, nearly all the borax of commerce is manufactured from 
colemanite. 

Colemanite (Ca 2 B 6 0n 5H2O) 

Colemanite occurs in crystals and in granular and compact masses. 
It is the source of all the borax now manufactured in the United States. 

The formula ascribed to the mineral corresponds to 27.2 per cent 
CaO, 50.9 per cent B2O3 and 21.9 per cent H2O. As usually found, 
however, it contains a little MgO and SK>2. A crystal from Death 
Valley, California, yielded: 

8203 = 50.70; CaO=27.3i; MgO=.io; H20=2i.87. Total = 99-98. 



NITRATES AND BORATES 



209 



The mineral crystallizes in the monoclinic system (prismatic class), 
in short, prismatic crystals (Fig. 104), with the axial constants a : b : c 
= .7769 : 1 : .5416 and £=69° 43'. The crystals are usually rich in 
forms. Their cleavage is perfect parallel to 00 O 00 (010), and less 
perfect parallel to oP(ooi). Their fracture is uneven. The angle 
110A 110=72° 4'. 

Colemanite is colorless, milky white, yellowish white or gray. It 
is transparent or translucent, has a vitreous or adamantine luster, a 
hardness of 4 to 4.5 and a specific 
gravity of 2.4. Its index of refrac- 
tion for yellow light, 0= 1.5920. 

Before the blowpipe it decrep- 
itates, exfoliates, and partially 
fuses, at the same time coloring 
the flame yellowish green. It is 
soluble in hot HC1, but from the 
solution upon cooling a volumi- 
nous mass of boric acid separates 
as a white gelatinous precipitate. Fig. 104. — Colemanite Crystals with « p, 

It is easily distinguished from IIO ( m >; 3P * , 301 (w); « P « , 100 (a); 





m 



m 




ooPjp,,oio (6); oP, 001 (c)\ — P, in 
(0); 2P«,o2i(a); Pob,oii(*c); co P2, 
210 (/); 2P06, 201 (A); 2P, 221 (*) and 
P, I" (y). 



other white translucent minerals, 
except those containing boron, 
by the flame test. It is distin- 
guished from borax by its insolu- 
bility in water and from boracite by its inferior hardness and crystal- 
lization. 

Synthesis. — Colemanite has been prepared by treating ulexite 
(NaCaB 5 9 • 8H2O) with a saturated solution of NaCl at 70 . 

Occurrence and Origin. — The mineral occurs as indefinite layers 
interstratified with shale and limestones that are associated with basalt. 
The rocks contain layers and nodules of colemanite. Gypsum is often 
associated with the borate and in some places is in excess. The cole- 
manite is believed to be the result of the action of emanations from 
the basalt upon the limestone. 

Localities. — Colemanite occurs in Death Valley, California, near 
Daggett, San Bernardino County, and near Lang Station, Los Angeles 
County, and at other points in the same State, and in western Nevada, 
near Death Valley. A snow-white, chalky variety (priceite) has been 
found in Curry County, Oregon, and a compact nodular variety (pander- 
mite) at the Sea of Marmora, and at various points in Asia Minor. 

Preparation. — Colemanite is at the present time the principal source 



210 DESCRIPTIVE MINERALOGY 

of borax. The crude material as mined contains from 5 per cent to 35 
per cent of anhydrous boric acid (B2O3). This is crushed and roasted. 
The colemanite breaks into a white powder which is separated from 
pieces of rock and other impurities by screening, and then is bagged and 
shipped to the refineries where it is manufactured into borax and boracic 
acid. 

Production. — The principal mines producing the mineral in 191 2 
were situated in the Death Valley section of Inyo County, near Lang 
Station in Los Angeles County, California, and in Ventura County in 
the same State. The total production during the year was 42,315 
tons of crude ore, valued at $1,127,813. The imports of crude ore, 
refined borax and boric acid during the same year were valued at $11,200. 
The production of the United States in boron acid compounds is 
about half that of the entire world, with Chile producing nearly all 
the rest. 

Boracite (Mg5(MgCl) 2 Bi 6 3 o) 

Boracite is interesting as a mineral, the form and internal structure 
of which do not correspond, that is, do not possess the same symmetry. 
Its crystals have the well marked hextetrahedral symmetry of the iso- 
metric system, but their internal structure, as revealed by their optical 
properties is orthorhombic. This is due to the fact that the substance 
is dimorphous. Above 265 ° it is isometric and below that temperature 
orthorhombic. Crystals formed at temperatures above 265° assume 
the isometric shapes. As the temperature falls the substance changes 
to its orthorhombic form, and there results a pseudomorph of ortho- 
rhombic boracite after its isometric dimorph. 

It is a salt of the acid which may be regarded as related to boric 
acid as follows: 8H3BO3— gl^C^HeBgOis. Ten atoms of hydrogen 
in two molecules of the acid are replaced by Mgs and the other two by 
2(MgCl). The resulting combination is: 31.4 per cent MgO; 7.9 per 
cent CI and 62.5 per cent B203=ioi.8(0=C1=i.9). The mineral 
alters slowly, taking up water, so that some specimens yield water on 
analysis and in the closed tube (stassfurti'.e and parasite). 

The forms usually found on the crystals are -(in), ooO(no), 

• 2 

00 O 00 (100), (1T1) (Fig. 105). Usually the positive and negative 

2 

tetrahedrons may be distinguished by their luster, the faces of the posi- 
tive form being brilliant and those of the negative form dull. The 
crystals are isolated, or embedded, and rarely in groups. They are 



NITRATES AND BORATES 



211 




Fig. 105. — Boracite 
Crystal with 00 O 00 , 
100 (a); 00 O, no (rf); 

O O 

+-, in (0) and --, 

ill (Oi). 



strongly pyroelectric with the analogue pole in the negative tetrahedrons. 
The mineral is also found massive. 

Boracite is transparent or translucent and is gray, yellow, or green. 
Its streak is white. Its luster is vitreous. Its cleavage is indistinct 
parallel to O(ni) and its fracture is conchoidal. 
The mineral is brittle. Its hardness is 7 and 
its density 3. Its refractive index 0, for yellow 
light, =1.667. 

Boracite fuses easily before the blowpipe 
with intumescence to a white pearly mass, at 
the same time coloring the flame green. With 
copper oxides it colors the flame azure-blue. 
When moistened with Co(NC>3)2 it gives the 
pink reaction for magnesium. Some massive 
forms yield water in the closed tube, in conse- 
quence of weathering. The mineral is soluble 
in HC1. 

Boracite is distinguished from other boron salts by its crystallization, 
its lack of cleavage and its much greater hardness. The massive vari- 
eties which resemble fine-grained white marble can be distinguished 
from this by the flame coloration, hardness and reaction with HC1. 

Syntheses. — Crystals have been formed by heating borax, MgCb 
and a little water at 275 , and by fusing borax with a mixture of NaCl 
and MgCb- 

Occurrence. — Boracite occurs in beds with anhydrite, gypsum and 
salt, and as crystals in metamorphosed limestones. 

Localities. — It is found as crystals in gypsum and anhydrite at 
Ltineburg, Hanover, and Segeberg, Holstein; in carnallite at Stassfurt, 
Prussia; and in radiating nodules (stassfurtite) and in massive layers 
associated with salt beds at the last-named locality. It is rare in the 
United States. 

Uses and Production. — Boracite is utilized in Europe as a source of 
boron compounds. Turkey produces annually about 12,000 tons. 



CHAPTER XI 
THE CARBONATES 

The carbonates constitute an important, though not a very large, 
group of minerals, though one of them, calcite, is among the most com- 
mon of all minerals. They are all salts of carbonic acid (H2CO3). Those 
in which all the hydrogen has been replaced by metal are normal salts, 
those in which the replacement has been by a metal and a hydroxyl 
group are basic salts. Both groups are represented by common minerals. 

The normal salts include both anhydrous salts and salts combined 
with water of crystallization. Illustrations of the three classes of car- 
bonates are: CaCC>3, calcite, normal salt; Na2C03 • 10H2O, soda, 
hydrous salt and (Cu '011)2003, malachite, basic salt. All carbonates 
effervesce in hot acids. The basic salts yield water at a high tempera- 
ture only; the hydrous ones at a low temperature. 

The carbonates are all transparent or translucent, and all are poor 
conductors of electricity. Most of them are practically nonconductors. 

ANHYDROUS CARBONATES 

NORMAL CARBONATES 

The anhydrous normal carbonates comprise the most important 
carbonates that occur as minerals. Most of them are included in a 
single large group whose members are dimorphous, crystallizing in the 
ditrigonal scalenohedral class of the hexagonal system and in the holo- 
hedral division (rhombic bipyramidal class) of the orthorhombic sys- 
tem. The calcium carbonate exists in three forms but only two are 
known to occur as minerals. 

CALCITE-ARAGONITE GROUP 

The relation of the dimorphs of this group to one another has been 
subjected to much study, especially with reference to the two forms of 
CaC03. The orthorhombic form, aragonite, passes into the hexagonal 
form, calcite, upon heating to about 400 . At all temperatures below 
970 , calcite is the stable form. Moreover, while calcite crystallizes 
from a dilute solution of CaC03 in water containing CP2 at a low tem- 

212 



CARBONATES 213 

perature, aragonite separates at a temperature approaching that of 
boiling water — the more freely, the less CO2 in the solution. Arag- 
onite crystals will also separate from a solution of calcium carbonate, 
if, at the same time, it contains a grain of an orthorhombic carbonate, 
or a small quantity of a soluble sulphate. Some of the other carbon- 
ates, for instance, strontianite (the orthorhombic SrCOs), pass over 
into an hexagonal form like that of calcite at 700°, but again change 
to the orthorhombic form upon cooling. For convenience the group 
is divided for discussion into the calcite division and the aragonite 
division. 

CALCITE DIVISION 

The calcite division of carbonates includes nine or more distinct 
compounds and a number of well defined varieties of these. Six of the 
compounds are common minerals. All crystallize in the ditrigonal 
scalenohedral class of the hexagonal system and are thus isomorphous. 
Their most common crystals have a rhombohedral habit. The names of 
the six common members with their axial ratios are: 

Calcite CaCCto : c= 1 : .8543 

Magnesite MgCC>3 =1 : .8095 

Siderite FeCC>3 =1 : .§191 

Rhodochrosite MnCC>3 =1 • 8259 

Smit'.iSonitc ZnCCfe =1 : .8062 

There is usually also included in the group the mineral dolomite, which 
is a calcium magnesium carbonate in which CaCCfe and MgCC>3 are 
present in the molecular proportions, thus: MgCCb-CaCOs, or 
MgCa(C03)2. Its crystals are similar to those of calcite and its physical 
properties are intermediate between those of calcite and magnesite. 
Its symmetry, however, as revealed by etching is tetartohedral (rhom- 
bohedral class). 

The close relationship existing between the members of the group 
(including dolomite) will be appreciated upon comparing the data in 
the following table: 

H 

Calcite 3. 

Dolomite 3-5~4 

Magnesite 3-5~4-5 

Siderite 3-5"4 

Rhodochrosite.. 3.5-4.5 

Smithsonite. ... 5. 









Ref. Indices 


Sp. Gr. 


a : c 


ion Aon 


I (1) 


c 


2-73 


■8543 


74° 55' 


1-6585 


1.4863 


2.85 


.8322 


73° 45' 


I. 6817 


1 . 5026 


3-°4 


.8095 


72° 36' 


1. 717 


I-5I5 


3-88 


.8191 


73° 0' 


1.8724 


16338 


3 55 


•8259 


73 


1.820 


1 5973 


4-45 


.8062 


72 20' 


i.8i8± 


1. 6177 



214 



DESCRIPTIVE MINERALUGY 



Calcite (CaC0 3 ) 

Calcite is one of the most beautifully jcrystallized minerals known. 
Its crystals are very common, and sometimes very large. They are 
usually colorless, though sometimes colored, and are nearly always 
transparent. Besides occurring in crystals the mineral is often found 
massive, in granular aggregates, in stalactites, in pulverulent masses, 





Fig. 106. 




e 



m 






Fig. 108. 



m 




Fig. 107. Fig. 109. 

Fig. 106.— Calcite Crystal with — JR, 01T2 (<?) and 00 R ; 10T0 (m). Nail-head Spar. 

Fig. 107. — Calcite Crystal with m and e. Prismatic Type. 

Fig. 108.— Calcite Crystals with m; R 3 , 2 131 (v) and R, 10T1 (r). Dog-tooth Spar. 

Fig. 109. — Calcite with r, v; 4R, 4041 (M) and R*, 3251 (y). 

in radial groupings, in fibrous masses and in a variety of other forms. As 
calcite is soluble in water containing CO2, it has often been found pseu- 
domorphing other minerals. 

Theoretically, calcite contains 56 per cent CaO and 44 per cent CO2, 
but practically the mineral contains also small quantities of Mg, Fe, 
Mn, Zn and Pb, metals whose carbonates are isomorphous with 
CaC0 3 . 

The forms that have been observed on calcite crystals are arranged 



CARBONATES 



215 



in such a manner as to produce three distinct types of habit, as fol- 
lows: (i) the rhombohedral type, bounded by the flat rhombohedrons, 
R(ioTi), — JR(oiTa) and often blunt scale- 
nohedrons, like R 3 (2ili) and ^(3145) 
in which the rhombohedrons predominate 
(Fig. 106); (2) the prismatic type, with 
the prism 00 P(ioTo) predominating, and 
— jR(oiia) as the principal termination 
(Fig. 107), and (3) dog-tooth spar, contain- 
ing the same scalenohedrons as on the first 
type mentioned above with other steeper 
ones and small steep rhombohedral planes 
(Fig. 108, 109, no). Nail-head spar con- 
tains the flat rhombohedron — JR(oiTa) 
with the prism 00 P(ioTo) (Fig. 106). 

Some of the crystals are very compli- 
cated, belonging to no one of the distinct 

types described above, but forming barrel-shaped or almost round 
bodies. Over 300 well established forms have been identified on them. 

Twins are common. The principal laws are: (1) twinning plane 
oP(oooi), with the vertical axis common to the twinned parts (Fig. 
in), (2) twinning plane — §R(otl2), with the two vertical axes inclined 



Fie. 1 10.— Prismatic Crystals 
of Calrite Terminated by 
Scalenohedrons and Rhom - 
bohedrons, from Cumber- 
laud, England. 




Fic. in. — Calcite, R* (.3131) Twinned about oP (0001). 
— Calcite: Twin and Per/synthetic Trilling of R (10T1) about — )R (01 



at an angle of about 525° (Fig. 112) and (3) twinning plane R(ioTi), 
with the vertical axes inclined 8o° 14' (Fig. 113). 

Twins of the second class can easily be produced artificially on cleav- 
age rhombs by pressing a dull knife edge on the obtuse rhombohedral 
edge with sufficient force to move a portion of the mass (Fig. 114). 
The change of position of a portion of the calcite does not destroy its 



DESCRIPTIVE MINERALOGY 



transparency in the least. Repeated twinning of this kind is frequently 
seen in marble (Fig. 115), where it gives rise to parallel lamellae. 

The cleavage of calcite is so perfect parallel to R that crystals when 




Fig. 



Fig. 113. F10. 114. 

Fig. 113— Calcite with m, c and e, Twinned about R (ioii). 
14.— Artificial Twin of Calcite, with — JR (011a) the Twinning Plane. 



shattered by a hammer blow usually break into perfect little rhombo- 
hedrons. Its hardness is about 3 and its density 2.713. Pure calcite 
is colorless and transparent, but most specimens are white or some pale 
shade of red, green, gray, 
blue, yellow, or even brown 
or black when very impure, 
and are translucent or opaque. 
The mineral is very strongly 
doubly refracting, (see p. 213). 
It is a very poor conductor of 
electricity. 

The principal varieties of 
the mineral to which distinct 
names have been given are: 

Iceland spar, the trans- 
parent variety used in the 
manufacture of optical instru- 
ments. 

Salin spar, a fine, fibrous 
variety with a satiny luster. 

Limestone, granular ag- 
gregates occurring as rock 
masses. 
Marble, a crystalline limestone, showing when broken the cleavage 
faces of the individual crystals. 

Lithographic stone. 1 very fine and even-grained limestone. 



Fig. 115. — Thin Section of Marble Viewed by 
Polarized Light. The dark bars are poly- 
synthetic twinning lamellae. Magnified 5 
diameters. 



CARBONATES 217 

Stalactites, cylinders or cones of calcite that hang from the roofs of 
caves. They are formed by the evaporation of dripping water. 

Stalagmites, corresponding cones on the floors of caves beneath the 
stalactites. 

Mexican onyx, banded crystalline calcite, often transparent. 
Usually portions of stalactites. 

Travertine, a deposit of white or yellow porous calcite produced 
in springs or rivers, often around organic material like the blades 
or roots of grass. 

Chalk, a fine-grained, pulverulent mass of calcite, occurring in 
large beds. 

In the closed tube calcite often decrepitates. Before the blowpipe it 
is infusible. It colors the flame reddish yellow and after heating reacts 
alkaline toward moistened litmus paper. The mineral dissolves with 
evolution of CO2 in cold hydrochloric acid. Its dissociation tempera- 
ture l is 898 , though it begins to lose CO2 at a much lower temperature. 

The reaction with HC1, together with the alkalinity of the mineral 
after heating, its softness and its easy cleavage, distinguish calcite from 
all other minerals. In massive forms it has been thought that it could 
be distinguished from aragonite by heating its powder with a little 
Co(NOa)2 solution. Aragonite was thought to become violet-colored 
in a few minutes while calcite remained unchanged, but recent work 
proves that this test cannot be relied upon. 

Syntheses. — Calcite crystals are obtained by allowing a solution 
of CaCOa in dilute carbonic acid to evaporate slowly in contact with the 
air at ordinary temperatures. If evaporated at from 8o° to ioo° 
ordinary temperatures, or in the presence of a little sulphate, the ortho- 
rhombic aragonite will form. Calcite is also formed by heating arag- 
onite to 400-470 . 

Occurrence and Origin. — The mineral is widely distributed in beds, 
in veins and as loose deposits on the bottoms of springs, lakes and rivers. 
Its principal methods of origin are precipitation from solutions, the 
weathering of calcareous minerals, and secretion by organisms. 

Calcite is the most important of all pseudomorphing agencies. It 
forms pseudomorphs after many different minerals and the hard parts 
of animals. 

Localities. — The most noted localities of crystallized calcite are: 
Andreasberg in the Harz; Freiberg, Schneeberg and other places in 
Saxony; Kapnik, in Hungary; Traverselia, in Piedmont; Alston Moor 

1 The dissociation temperature of a carbonate is that temperature at which the 
pressure of the released CO? equals one atmosphere. 



218 DESCRIPTIVE MINERALOGY 

and Egremont, in Cumberland, Matlock, in Derbyshire, and the mines 
of Cornwall, England; Guanajuato, Mexico; Lockport, N. Y.; Ke- 
weenaw Point, Mich.; the zinc regions of Illinois, Wisconsin and 
Missouri; Nova Scotia, etc. 

Iceland spar is obtained in the Eskefjord and the Breitifjord in 
Iceland. Travertine is deposited from the waters of the Mammoth 
Hot Springs, Yellowstone National Park. It occurs also along the 
River Arno, near Tivoli, Rome. 

Uses. — Calcite has many important uses. In the form of Iceland 
spar, on account of its strong double refraction, it is employed in optical 
instruments for the production of polarized light. Calcite rocks are 
used as building and ornamental stones. They are employed also as 
fluxes in smelting operations, as one of the ingredients in glass-making 
and in the manufacture of lime, cement, whiting, and in certain printing 
operations. Limestone is also used as a fertilizer. 

Production. — The calcite rock marketed in the United States during 
191 2 was valued at about $44,500,000. It was used as follows: In 
concrete, $5,634,000; in road and railroad making, $12,000,000; as a 
flux, $10,000,000; as building and monumental stone, $12,800,000; 
in sugar factories, $335,000; as riprap, $1,183,000, for paving, $279,000, 
and for other uses, $2,400,000. Moreover, the value of the Portland 
cement manufactured during the year amounted to $67,017,000, the 
quantity of lime made to $13,970,000, the value of the hydrated 
lime to $1,830,000, and of sand-lime brick to $1,170,884. The quantity 
of limestone required for these manufactures is not known, but it was 
very great. 

Magnesite (MgC0 3 ) 

Magnesite usually occurs in fine-grained white masses. Crystals 
are rare. Pure magnesite consists of 52.4 per cent CO2 and 47.6 per 
cent MgO. It usually, however, contains some iron carbonate. 

Magnesite is completely isomorphous with calcite. Its cleavage is 

perfect parallel to R(ioTi). Its hardness is about 4 and the density 3.1. 

The mineral is transparent or opaque. It varies in color from white 

to brown, but always has a white streak. Its dissociation temperature 

is 445 . 

Magnesite behaves like calcite before the blowpipe. It effervesces 
in hot hydrochloric acid and readily yields the reaction for magnesia 
with Co(N03)2. It is most easily distinguished from the latter mineral 
by its density, by the fact that it does not color the blowpipe flame with 
the yellowish red tint of calcium and does not effervesce in cold HC1. 



CARBONATES 219 

Synthesis. — Magnesite crystals may be obtained by heating MgSOi 
in a solution of Na2CC>3 at 160 in a closed tube. 

Occurrence and Origin. — Magnesite usually occurs in veins and masses 
associated with serpentine and other magnesium rocks from which it 
has been formed by decomposition. It is often accompanied by brucite, 
talc, dolomite and other magnesium compounds. It has recently been 
described as occurring also in a distinct bed near Mohave, Cal., inter- 
stratified with clays and shales. It is thought that in this case it may 
have been precipitated from solutions of magnesium salts by Na2C03. 

Localities. — The mineral is found abundantly in many foreign local- 
ities and at Bolton, Mass.; Bare Hills, near Baltimore, Md., and in 
Tulare Co., Cal., and near Texas, Penn. The largest deposits are in 
Greece and Hungary. 

Uses. — Magnesite is employed very largely in the manufacture of 
magnesite bricks used for lining converters in steel works, in the lining 
of kilns, etc., in the manufacture of paper from wood pulp, and in mak- 
ing artificial marble, tile, etc. From it are also manufactured epsom 
salts, magnesia (the medicinal preparation), and other magnesium com- 
pounds, and the carbon dioxide used in making soda water. 

Production. — All of the magnesite mined in the United States comes 
from California, where the yield was 10,512 tons in 191 2, valued at 
$105,120. Most of the magnesite used in the United States is imported 
from Hungary and Greece. In 191 2, 14,707 tons of crude material 
entered the country and 125,000 tons of the calcined product, the total 
value of which was $1,370,000. 

Siderite (FeC0 3 ) 

Siderite is an important iron ore, though not as much used as formerly. 
It is found crystallized and massive, in botryoidal and globular forms 
and in earthy masses. 

In composition the mineral is FeCC>3, which is equivalent to 62.1 
per cent FeO (48.2 per cent Fe) and 37.9 per cent CO2. Manganese, 
calcite and magnesium are also often present in it. 

Crystals are more common than those of magnesite. They fre- 
quently contain the basal plane and the steep rhombohedrons— 8R(o85i) 
and — 58.(0551). R(ioTi) and — £R(oil2) are common. The faces 
of the rhombohedron are frequently curved. Compare (Fig. 125.) 

The cleavage of siderite is like that of the other minerals of this 
group. Its hardness is 3.5-4 and density 3.85. In color the mineral 
is sometimes white, but more frequently it is some shade of yellow or 
brown. Its streak is white. Most specimens are translucent. 



220 DESCRIPTIVE MINERALOGY 

In the closed tube siderite decrepitates, blackens and becomes mag- 
netic. It is only slowly affected by cold acids but it effervesces briskly 
in hot ones. 

Siderite is distinguished from the other carbonates by its reaction 
for iron. 

The mineral changes on exposure into limonite and sometimes into 
hematite or even into magnetite. 

Synthesis. — Crystals of siderite may be obtained by heating a solu- 
tion of FeSC>4 with an excess of CaCC>3 at 200 . 

Occurrence and Origin. — The mineral is often found accompanying, 
metallic ores in veins. It occurs also as nodules in certain clays and in 
the coal measures. In some cases it appears to be a direct deposit from 
solutions. In others it is a result of metasomatism and in others is an 
ordinary weathering product. 

Localities. — The crystallized variety is found at Freiberg, in Saxony; 
at Harzgerode, in the Harz; at Alston Moor, and in Cornwall, Eng- 
land; and along the Alps, in Styria and Carinthia. Cleavage masses 
are present in the cryolite from Greenland. 

Workable beds of the ore are present in Columbia Co., and at Rossie, 
in St. Lawrence Co., N. Y.; in the coal regions of Pennsylvania and 
Ohio, and in clay beds along the Patapsco River, in Maryland. The 
massive or nodular ore from clay banks is known as ironstone. The 
impure bedded siderite interstratified with the coal shales is known 
as black-band ore. 

Production. — Only 10,346 tons of siderite were produced in the United 
States during 191 2, all of it coming from the bedded deposits in Ohio. 
This was valued at $20,000. 

Rhodochrosite (MnC0 3 ) 

This mineral sometimes occurs in distinct crystals of a rose-red 
color; but it is usually found in cleavable masses, in a compact form, 
or as a granular aggregate. Sometimes it is in incrustations. It is 
not of commercial importance in North America. 

Pure manganese carbonate containing 61.7 per cent MnO and 38.3 
per cent CO2 is rare. The mineral is usually impure through the addi- 
tion of the carbonates of iron, calcium, magnesium or zinc. 

The most prominent forms on crystals of rhodochrosite are R(io7i), 
— £R(oil2), ooP2(ii2o), oR(oooi) and various scalenohedrons. 

Its cleavage is perfect parallel to R. The mineral is brittle. Its 
hardness is about 4 and its density about 3.55. Its luster is vitreous, 
and its color red, brown, or yellowish gray. Its streak is white. When 



CARBONATES 221 

heated it begins to lose CO2 at about 320 ; but its dissociation temper- 
ature is 63 2°. 

The mineral is infusible, but when heated before the blowpipe it 
decrepitates and changes color. When treated in the borax bead it 
gives the violet color of manganese, and when fused with soda on char- 
coal it yields a bluish green manganate. It dissolves in hot hydro- 
chloric acid. 

There are bu.t few minerals resembling pure rhodochrosite in appear- 
ance. From all of these, except the silicate, rhodonite (p. 380), it is 
distinguished by its reaction for manganese. It is distinguished from 
rhodonite by its hardness, its cleavage and its .effervescence with acids. 
The impure varieties are very like some forms of siderite, from which, 
of cqurse, the manganese test will distinguish it. 

Synthesis. — Small rhombohedrons of rhodochrosite have been pro- 
duced by heating a solution of MnSO* with an excess of CaCCfe at 200 
in a closed tube. 

Occurrence and Origin. — Rhodochrosite occurs in veins associated 
with ores of silver, lead, copper and other manganese ores and in bedded 
deposits. It is the result of hydrothermal or contact metamorphism, 
and of weathering of other manganese-bearing minerals. 

Localities. — The mineral is found at Schemnitz, in Hungary; at 
Nagyag, in Transylvania; at Glendree, County Clare, Ireland, where it 
forms a bed beneath a bog; at Washington, Conn., in a pulverulent 
form; at Franklin, N. J.; at the John Reed Mine, Aliconte, Lake Co., 
and at Rico, Colo.; at Butte City, Mont.; at Austin, Nev., and on 
Placentia Bay, Newfoundland. The Colorado and Montana specimens 
are well crystallized. 

Uses. — The mineral is mined with other ores of manganese. Occa- 
sionally it is employed as a gem stone. 

Smithsonite (ZnCOs) 

Smithsonite, or " dry-bone ore," is rarely well crystallized. It 
appears as druses, botryoidal and stalactitic masses, as granular aggre- 
gates and as a friable earth. 

In ZnC03 there are 64.8 per cent ZnO and 35.2 per cent CO2. Smith- 
sonite usually contains iron and manganese carbonates, often small 
quantities of calcium and magnesium carbonates and sometimes traces 
of cadmium. A specimen from Marion, Arkansas, gave: 



ZnO 


CdO FeO 


CaO 


CuO 


C0 2 


CdS 


Si0 2 


Total 


64.12 


.63 .14 


.38 


tr. 


34.68 


•25 


.06 


100.26 



222 DESCRIPTIVE MINERALOGY 

The mineral is closely isomorphous with calcite, R(ioTi), — JR(oiT2), 
4R(404i), oo R2(ii2o), oR(oooi) and R 3 (2i3i) being present on many 
crystals. The R faces are rough or curved. 

Its cleavage is parallel to R(ioTi). Its hardness is 5 and its density 
about 4.4. The luster of the mineral is vitreous, its streak is white and 
its color white, gray, green or brown. It is usually translucent, occa- 
sionally transparent. When heated to 300 for one hour it loses all of 
its C0 2 . 

When heated in the closed tube CO2 is driven off, leaving ZnO as a 
yellow residue while hot, changing to white on cooling. The mineral 
is infusible before the blowpipe. If a small fragment be moistened with 
cobalt nitrate solution and heated in the oxidizing flame it becomes 
green on cooling. When heated on charcoal a dense white vapor is 
produced. This forms a yellow coating on the coal, which, when it 
cools, turns white. If this be moistened with cobalt nitrate and reheated 
in the oxidizing flame it is colored green. 

The above reactions for zinc, together with the effervescence of the 
mineral in hot hydrochloric acid distinguish smithsonite from all other 
compounds. 

Smithsonite forms pseudomorphs after sphalerite and calcite and is 
pseudomorphed by quartz, limonite, calamine and goethite. 

Synthesis. — Microscopic crystals of smithsonite may be produced by 
precipitating a zinc sulphate solution with potassium bicarbonate and 
allowing the mixture to stand for some time. 

Occurrence. — Smithsonite occurs in beds and veins in limestones, 
where it is associated with galena and sphalerite and usually with cala- 
mine (p. 396). It is especially common in the upper, oxidized zone of 
veins of zinc ores and as a residual deposit covering the surface of weath- 
ered limestone containing zinc minerals. 

Localities. — The mineral is found at Nerchinsk, Siberia; Bleiberg, 
in Carinthia; Altenberg, Aachen; Province of Santander, Spain; at 
Alston Moor and other places in England; at Donegal, in Ireland; at 
Lancaster, Penn.; at Dubuque, Iowa; in Lawrence and Marion Coun- 
ties, Arkansas; and in the lead districts of Wisconsin and Missouri (see 
galena and sphalerite). 

The Wisconsin and Missouri localities are the most important ones 
in North America. Here the ore occurs in botryoidal, in stalactitic 
and in earthy, compact, cavernous masses of a dull yellow color incrusted 
with druses of smithsonite crystals, of calamine and of other minerals, 
principally of lead. This is the variety known as " dry bone." 

Uses. — The mineral was formerly an important ore of zinc, being " 



CARBONATES 223 

mined alone for smelting. It is now mined only in connection with 
calamine and other zinc ores, and all are worked up together. A trans- 
lucent green or greenish blue variety occurring at Laurium, Greece, 
and at Kelly, New Mexico," is sometimes employed for ornamental pur- 
poses. About $650 worth of the material from New Mexico was utilized 
as gem material in 191 2. 

ARAGONITE DIVISION 

This division of the carbonates includes the orthorhombic (rhombic 
bipyramidal) dimorphs of the members of the calcite group which, 
together, form a well characterized isodimorphous group. The carbon- 
ate of calcium is found well crystallized in both divisions, but the other 
carbonates are common to one only. They actually occur in both divi- 
sions, but they are found as common members of one and only as 
isomorphous mixtures with other more common forms in the other. 
Thus, barium carbonate is a common orthorhombic mineral under the 
name of uitherite. It occurs also with CaCC>3 hi mixed crystals under 
the name baricalcite, or neotype y which is hexagonal. (See also p. 212 
and p. 213.) 

The common members of the aragonite division are: 

Aragonite CaCC>3 Sp. Gr. = 2.936 a : b : c= .6228 : 1 

Stroniianite SrC03 = 3 . 706 = . 6090 : 1 

Witherite BaC(>3 =4.325 = . 5949 : 1 

Cerussite PbC03 =6.574 =.6102:1 



.7204 
.7266 

•7413 
.7232 



Aragonite (CaCOs) 



Aragonite occurs in a great variety of forms. Sometimes it is in 
distinct crystals, but more frequently it is in oolitic globular and reni- 
form masses, in divergent bundles of fibers or of needle-like forms, in 
stalactites and in crusts. 

In composition aragonite is like calcite. It often contains small 
quantities of the carbonates of strontium, lead or zinc. 

Crystals are often acicular with steep domes predominating. Some 
of the simplest crystals consist of ooP(no), 00P06 (010), fPoo (032), 
P06 (on), 4P(44i), 9P(99i) and ooP2(i2o) (Fig. 116). Twinning is 
common. The twinning plane is often ooP(no). By repetition this 
gives rise to pseudohexagonal forms, resembling an hexagonal prism and 
the basal plane (see Figs. 117 and 118). The angle noAiTo=63° 48'. 

The cleavage of aragonite is distinct parallel to 00 P 06 (010) and 
indistinct parallel to 00 P(no). Its hardness is 3.5-4 and density about 
2.93. Its luster is vitreous and its color white, often tinged with gray, 



DESCRIPTIVE MINERALOGY 



green or some other light shade. Its streak is white and the mineral is 
transparent or translucent. Its indices of refraction for yellow light are: 
a= 1.5300, 7= 1.6857. At 400 it passes over into calcite. 

Before the blowpipe aragonite whitens and falls to pieces. Other- 
wise its reactions are like those of calcite, from which it can be distio- 



^P\ 



kj>^ 




/ 1 1 u 1. in a 

u m » 1 i m B 



Fie. 116.— Aragonite Crystal with =oP,iio(m); »Pw, 010 (S) and P« , on {*). 
Fig. 117. — Aragonite Twin and Trilling Twinned about » P (no). 




Fie. 118.— Trilling of Aragonite Twinned about »P (no). (/I) Cross-section. 
(B) Resulting pseudohexagonal group, resembling an hexagonal prism and 

guished by its crystallization, its lack of rhombohedral cleavage and its 
density. 

Synthesis. — Solutions of OCO3 in dilute H2CO3 form crystals of 
aragonite when evaporated at a temperature of about 90 . In general, 
hot solutions of the carbonate deposit aragonite, while cold solutions 
deposit calcite. If the solution contains some sulphate or traces of 
strontium or lead carbonates, mixed crystals consisting principally of 
the aragonite molecule are formed at ordinary temperature. 

Occurrence and Origin. — Aragonite occurs in beds, usually with 
gypsum. It is also deposited from hot waters and from cold waters 



CARBONATES 225 

containing a sulphate (as from sea water). The pearly layer of oyster 
shells and the body of the shells of some other mollusca are composed 
of calcium carbonate crystallizing like aragonite. Aragonite is often 
changed by paramorphism into calcite, pseudomorphs of which after 
the former mineral are quite common. 

Localities. — The mineral is found at Aragon, Spain; at Bilin, in 
Bohemia; in Sicily; at Alston Moor, England; and at a number of 
other places in Europe. It occurs in groupings of interlacing slender 
columns (flos ferrt) , in the iron mines of Styria. Stalactites are abundant 
at Leadhills, Lanarkshire, Scotland, and a silky fibrous variety known as 
satins par, at Dayton, England. 

In the United States crystallized aragonite occurs at Mine-la-Motte, 
Mo., and in the lands of the Creek Nation, Oklahoma. Flos ferri has 
been reported from the Organ Mts., New Mexico, and fibrous masses 
from Hoboken, N. J., Lockport, Edenville and other towns in New York 
and from Warsaw, 111. 

Strontianite (SrC0 3 ) 

In general appearance and in its manner of occurrence strontianite 
resembles aragonite. Its crystals are often acicular in habit though 
repeated twins are common. The angle noAiTo=62° 41'. 

The composition of pure strontianite is SrO=7o.i, ^2=29.9, but 
the mineral usually contains an admixture of the barium and calcium 
carbonates. 

Strontianite is brittle, its hardness is 3.5-4 and its density 3.7. 

Before the blowpipe strontianite swells and colors the flame with a 
crimson tinge. It dissolves in hydrochloric acid. The solution im- 
parts a crimson color to the blowpipe flame. When treated with sul- 
phuric acid it yields a precipitate of SrSO*. Its refractive indices for 
yellow light are: a =1.5 199, 7=1.668. Its dissociation temperature is 

"55°. 

Aragonite, witherite (BaCOa) and strontianite are so similar in ap- 
pearance and in general properties that they can be distinguished from 
one another best by their chemical characteristics. They are all sol- 
uble in hydrochloric acid and these solutions impart distinctive colors 
to the blowpipe flame (see p. 477). 

Syntheses. — Crystals of strontianite are obtained by precipitating 
a hot solution of a strontium salt by ammonium carbonate, and by cool- 
ing a solution of SrC03 in a molten mixture of NaCl and KC1. 

Occurrence. — Strontianite occurs in veins in limestone and as an 



226 DESCRIPTIVE MINERALOGY 

alteration product of the sulphate (celestite) where this is exposed to the 
weather. It is probably in all cases a deposit from water. 

Localities. — Strontianite is the most common of all strontiaa-fiem- 
goupds. It frequently occurs as the filling of metallic veins. It forms 
finely developed crystals at the Wilhelmine Mine near Munster, West- 
phalia. At Schoharie, N. Y., it occurs as crystals and as granular masses 
in nests in limestone. It is found also at other places in New York, in 
Mifflin Co., Penn., and on Mt. Bannell near Austin, Texas. 

Uses. — Strontium compounds are little used in the arts. The 
hydroxide is employed to some extent in refining beet sugar and the 
nitrate in the manufacture of " red fire." Other compounds are used 
in medicine. All the strontium salts used in the United States are 
imported. 

Witherite (BaC0 3 ) 

Witherite differs very little in appearance or in manner of occurrence 
from aragonite. Its crystals are nearly always in repeated twins that 

have the habit of hexagonal pyramids' (Fig. 
119). The angle 1 10 A 1 To = 62° 46'. 

When pure the mineral contains 77.7 per 
cent BaO and 22.3 per cent CO2. 

It is much heavier than the calcium car- 
bonate, its density being 4.3. Its hardness 
Fig. 119.— Witherite Twinned is 3 to 4. Its refractive index for yellow 
about cop (1 10), thus Imi- Hghtj /3=Ii740 . i ts dissociation tempera- 

tating Hexagonal Combina- . 

tions. m °° 

It dissolves readily in dilute hydrochloric 

acid with effervescence, and from this solution, even when dilute, sul- 
phuric acid precipitates a heavy white precipitate of BaS04, which, 
when heated in the blowpipe flame, imparts to it a yellowish green 
color. 

Witherite is distinguished from the other carbonates by its crys- 
tallization, and the color it imparts to the blowpipe flame. 

Syntheses. — Crystals are produced by precipitating a hot solution of 
a barium salt with ammonium carbonate, and by cooling a molten 
magma composed of NaCl and BaC03. 

Localities. — Witherite is not a very common mineral in the United 
States, but it occurs in large quantity associated with lead minerals in 
veins at Alston Moor, in Cumberland and near Hexham, in Northum- 
berland, England. Some of the crystals found in these places measure 
as much as six inches in length. 





CARBONATES 



227 



Its best known locality in the United States is Lexington, Kentucky, 
where the mineral is associated with the sulphate, barite. 

Uses. — It is used to some extent as a source of barium compounds. 
The importations of the mineral during 1912 aggregate $25,715. 

Cerussite (PbC0 3 ) 

Cemssite generally occurs in crystals and in granular, earthy and 
fibrous masses of a white color. 

The pure lead carbonate contains COa=i6.5 and PbO=83.5, bui 
the mineral usually contains in addition some ZnCOs. 




Mi -PS, «©<»); 
<*); JP», onto and 



—Cerussite Trilling Twinned about °°P(uo). 
—Cerussite Trilling Twinned about « P3(l3o). 



Its simple crystals are tabular combinations of °oP(no}, 00 Pd6 (010) 
ooPob(roo) and various brachydomes (Fig. 120), and these are often 
twinned in such a way as to produce six rayed stars (Fig. 121), or other 
symmetrical forms (Fig. 122). Groups of interpenetrating crystals 
are also common. The angle iioaii°=o2° 46'. 

The color of the mineral is usually white, but its surface is frequently 
discolored by dark decomposition products. Its luster is adamantine 
or vitreous and its hardness is 3-3.5. Itsdensity=6.s. Its refractive 
indices for yellow light are: = 1.8037,^=2.0763,7=2.0780. 

The mineral is dissolved by nitric acid with effervescence and by 
potassium hydroxide. Before the blowpipe it decrepitates, turns yellow 
and changes to lead oxide. On charcoal it is reduced to a metallic 
globule, and yields a white and yellow coating. 



228 DESCRIPTIVE MINERALOGY 

Cerussite is not easily confused with other minerals. It is well char- 
acterized by its high specific gravity, its reaction (or lead, and is dis- 
tinguished from the sulphate (anglestie) by effervescence with hot acids. 

Syntheses. — Crystals have been obtained by heating lead formate with 
water in a closed tube, and by treatment of a lead salt by a solution of 
ammonium carbonate at a temperature of tso°-i8o°. 

Occurrence and Origin. — The mineral occurs at all localities at which 
other lead compounds are found, since it is often produced from thes? 



Fig. 123. — Radiate Groups of Cerussite on Galena from Part City District, Utah. 
(After J. M. BoutmB.) 

latter by the action of the atmosphere and atmospheric water. It is, 
therefore, usually found in the upper portions of veins. 

Localities. — Cerussite crystals of great beauty are found in many of 
the lead-producing districts of Europe and also at Phoenixville, Penn.; 
near Union Bridge, in Maryland; at Austin's Mines, Wythe Co., Vir- 
ginia, and occasionally in the lead mines of Wisconsin and Missouri. 
In the West it occurs at Leadville, Colo.; at the Flagstaff and other 
mines in Utah (Fig. 123), and at several different mines in Arizona. 

Uses.— It is mined with other lead compounds as an ore of the metal. 



CARBONATES 229 

Dolomite (MgCa(C0 3 ) 2 ) 

Dolomite is apparently isomorphous with calcite but the etch 
figures on rhombohedral faces prove it to belong in the trigonal 
rhombohedral class. It occurs as crystals and in all the forms charac- 
teristic of calcite except the fibrous. 

Nearly all calcite contains more or less magnesium carbonate, but 
most of the mixtures are isomorphous with calcite and magnesite. 
When the ratio between the two carbonates reaches 54.35 per cent 
CaC03 : 45.65 per cent MgCC>3, which is equal to the ratio between 
the molecular weights of the two substances, or in other words when the 
two carbonates are present in the compound in the ratio of one molecule 
to one molecule, the mineral is called dolomite. The calculated com- 
position of dolomite (MgCa(COs)2) is 30.4 per cent CaO; 21.7 per cent 
MgO and 47.8 per cent CO2. 

The crystals of dolomite are usually rhombohedral combinations of 
the rhombohedron R(ioTi) with the scalenohedron 
R 3 (2i3i) (Fig. 124), and its tetartohedral forms, 
and often the prism ooP2(ii2o) and the basal 
plane. Its axial ratio is a : c=i : .8322. Twins 
are not rare, with oR(oooi) and R(ioTi) the 
twinning planes. The R planes are often curved, 
frequently with concave surfaces (Fig. 125). The 
angle 10T1 A 1101 = 73°. FiG l2 _ Dolomite 

The cleavage of dolomite is perfect parallel crystal with° R 
to R. The mineral is brittle. Its hardness is 40 ^ I (^ an( j p' 
3.5-4 and density 2.915. Its luster is vitreous or 0001 (c). 
pearly and its color white, red, green, gray or 
brown. Its streak is always white and the mineral is translucent or 
transparent. Its refractive indices for yellow light are: a>=i.68i7, 
e= 1.5026. The important varieties recognized are: 

Pearlspar, with curved faces having a pearly luster. 

Granular or saccharoidal, including many marbles and magnesian 
limestones. 

Dolomitic limestone, including much hydraulic limestone. 

Many dolomites are intermixed with the carbonates of iron, manga- 
nese, cobalt or zinc and these are known as ferriferous dolomite, etc. 

Dolomite behaves like calcite before the blowpipe and in the closed 
tube. It, however, dissolves only slowly, if at all, in cold hydrochloric 
acid, except when very finely powdered, though dissolving readily with 
effervescence in hot acid. 




230 DESCRIPTIVE MINERALOGY 

The reaction toward cold acid and its greater hardness easily dis- 
tinguish dolomite from catcUe. It is distinguished from magnetite by 
the name reaction. 

Occurrence and Origin. — Dolomite, like the calcium carbonate, occurs 
crystallized in veins, and as granular masses forming great beds of rock. 
It is a precipitate from solutions and a metasomatic alteration product 
of calcite. 

Localities. — Its crystals are present at many places, among them 
Bex, in Switzerland; Traversella, in Piedmont; Guanajuato, in Mexico; 
Roxbury, in Vermont; Hoboken, N. J.; Niagara Falls, the Quarantine 



Fig. 135. — Group of Dolomite Crystals from Joplin, Mo. Flat Rhombobcdrons with 
Curved Faces. 

Station, and Putnam, N. V.; Joplin, Mo.; and Stony Point, N. C. It 
is also very widely spread as beds of dolomitic limestone. 

Uses. — Dolomite is used for many of the purposes served by calcite; 
indeed, much of the material used as marble, limestone, etc., contains a 
large percentage of magnesium carbonate. It is not, however, used as a 
flux or in the manufacture of Portland cement, nor as a source of lime. 

Ankerite(Ca(Mg-Fe)(C0 3 )2) is a ferruginous dolomite. It is an 
isomorphous mixture of the carbonates of calcium, magnesium and iron, 
in which the FeC03 replaces a part of the MgCOs in dolomite. It is 
usually in rhombohedral crystals, with the angle 10T1 aTioi = 73° 48'. 
Its color is white, gray or red and its streak is white. Its hardness 
=3.5-4, and its density = 2.98. It also occurs in coarse and fine-grained 
granular masses. Ankerite is infusible before the blowpipe. In the 



CARBONATES 231 

closed tube it darkens and when heated on charcoal it becomes mag- 
netic. It occurs in veins, especially those containing iron minerals. 
It has been found at Antwerp and other places in northern New York. 

CALCIUM-BARIUM CARBONATES 

Carbonates of the general composition CaBa(COs)2 occur (i) as a 
series of mixed crystals isomorphous with calcite under the name bari- 
calcite; (2) as a series of mixed crystals isomorphous with aragonite 
known as alstonite or bromlite, and (3) a typical double salt, barytocalcite, 
which is monoclinic. Both alstonite and barytocalcite occur in veins 
of lead ores and of barite (BaSC>4). 

Barytocalcite, CaBa(COs)2 is monoclinic (prismatic class), with 
a : b : £=.7717 : 1 : 6255 and $ = 73° 52'. It forms crystals bounded 
by 00 P 6b (100), ooP(no), oP(ooi), and a series of clinopyramids, of 
which 2P2 (1 2!) and 5P5 (1 5!) are common. It also occurs massive. Its 
perfect cleavage is parallel to ooP(no). The mineral is white, gray, 
greenish or yellowish. Its streak is white, hardness =4 and sp. gr.= 
3.665. It is transparent or translucent. Before the blowpipe frag- 
ments fuse on thin edges, and assume a pale green color, due to the 
presence of a little manganese. The mineral is soluble in HC1. Its 
principal occurrence is Alston Moor, Cumberland, England. 

BASIC CARBONATES 

The basic carbonates are salts in which all or a portion of the hydro- 
gen of carbonic acid is replaced by the hydroxides of metals. There 
are only three minerals belonging to the group that need be referred to 
here. Two are copper compounds. One is the bright green malachite 
and the other the blue azurite. The composition of the former may be 

CuOHv 
represented by the formula 7CO3, and that of the latter by 

CuOH' 
CuOHv 

Cu=(COs)2. Both are used to some extent as ores of the metal, 
CuOH/ 

though their value for this purpose is not great at the present time. 
They may easily be distinguished from all other minerals by their 
distinctive colors, by the fact that they yield water in the closed tube 
and by their effervescence with acids. The third mineral (hydrozincite) 
is a white substance that occurs as earthy or fibrous incrustations on other 
zinc compounds. Its composition corresponds to 2ZnC03*3Zn(OH)2. 



232 



DESCRIPTIVE MINERALOGY 



Its hardness = 2-2.5 an ^ * ts specific gravity is about 3.7. Only the two 
copper compounds are described in detail. 



Malachite ((CuOH) 2 C0 3 ) 

Malachite usually occurs in fibrous, radiate, stalactitic, granular 
or earthy, green masses, or as small drusy crystals covering other copper 
compounds. The mineral contains, when pure, 19.9 per cent CO2, 
71.9 per cent CuO and 8.2 per cent H2O. 

Well defined crystals are usually very small monoclinic prisms (mon- 

oclinic prismatic class), with an axial ratio .8809 : 1 
: .4012 and 0=6i° 50'. Their predominant forms 
are 00 Pco (100), 00 Pod (010), ooP(no), and 
oP(ooi). Contact twins are common, with 
00 Pco (100) the twinning plane (Fig. 126). The 
angle no A 1^0=75° 40'. 

The pure mineral is bright green in color and has 
a light green streak. It possesses a vitreous luster, 
but this becomes silky in fibrous masses and dull 
in massive specimens. Crystals are translucent 
and massive pieces are opaque. Translucent 




Fig. 126. — Malachite 
Crystal with 00 P, 
no (m); 00 Poo, 

100 (o), and oP, pieces are pleochroic in yellowish green and dark 
001 (c) Twinned g reen tmts f ne deavage is perfect parallel to 

oP(ooi). The hardness of malachite is 3.5-4, and 
its density about 3.9. Its refractive index, /?, for yellow light=i.88. 

Malachite turns black and fuses before the blowpipe and tinges the 
flame green. With Na2C03 on charcoal it yields a copper globule. It is 
difficultly soluble in pure water, but is easily dissolved in water con- 
taining CO2. It is soluble with effervescence in HC1 and its solution 
becomes deep blue on the addition of an excess of ammonia. When 
heated in a closed glass tube, it gives an abundance of water. Boiled 
with water it turns black and loses its CO2. 

Malachite, on account of its characteristic color, may be easily dis- 
tinguished from all other minerals but some varieties of turquoise and 
a few copper compounds, such as atacamile (p. 144). It may be dis- 
tinguished from all of these by its effervescence with acids. 

Synthesis. — Malachite crystals have been obtained with the form of 
natural crystals by heating a solution of copper carbonate in ammonium 
carbonate. 

Occurrence and Origin. — Malachite is a frequent decomposition 
product of other copper minerals, being formed rapidly in moist places. 



CARBONATES 233 

It occurs abundantly in the upper oxidized portions of veins of copper 
ore, where it is associated with azurite, cuprite, copper, limonite and the 
sulphides of iron and copper, often pseudomorphing the copper minerals. 
The green stain noticed on exposed copper trimmings of buildings is 
composed in part of this substance. 

Localities. — The mineral occurs in all copper mines. At Chessy, 
France, it forms handsome pseudomorphs after cuprite. In the United 
States it has been found in good specimens at Cornwall, Lebanon Co., 
Penn.; at Mineral Point, Wisconsin; at the Copper Queen Mine, Bisbee, 
and at the Humming Bird Mine, Morenci, Arizona, and in the Tintic 
district, Utah. 

Uses. — In addition to its use as an ore of copper the radial and mass- 
ive forms of malachite are employed as ornamental stones for inside 
decoration. The massive forms are also sawn into slabs and polished 
for use as table tops and are turned into vases, etc. 

Production. — As malachite is mined with other copper compounds, 
the quantity utilized as an ore of the metal is not known. The amount 
produced in the United States during 191 2 for ornamental purposes was 
valued at $1,085. This, however, included also a mixture of malachite 
and azurite. 

Azurite (Cu(CuOH) 2 (C0 3 )2) 

Azurite is more often found in crystals than is malachite. It occurs 
also as veins and incrustations and in massive, radiated, and earthy 





Fig. 127. — Azurite Crystals with oP, 001 (c); — Poo, 101 (<r); 00 Poo, 100 (a); 
P, In (*), 00 P, no (w); -2P, 221 (A); JP2, 243 (d) and Poo , on (/). 

forms associated with malachite and other copper compounds. Its 
most frequent associate is malachite, into which it readily alters. 

In composition azurite is 25.6 per cent CO2, 69.2 per cent CuO, and 
5.2 per cent H2O. It changes rapidly to malachite, and sometimes is 
reduced to copper. 

The crystals are tabular, prismatic, or wedge-shaped monoclinic 
forms (monoclinic prismatic class), with an axial ratio a : b : £=.8501 : 
1 : 1.7611, and £=87° 36'. They are usually highly modified, 58 or 



234 DESCRIPTIVE MINERALOGY 

more different planes having been identified on them. The predominant 
ones are oP(ooi), — P<»(ioi), oo P(no), — 2P(22i) and ooPao(ioo). 
(Fig. 127.) The angle noAiTo=8o° 40'. 

The mineral is dark blue, vitreous, and translucent or transparent, 
and is pleochroic in shades of blue. It is brittle. Its streak is light 
blue, its hardness 3.5-4 and density 3.8. Its cleavage is distinct parallel 
to Pob (on). 

The blowpipe and chemical reactions for azurite are the same as 
those for malachite. By them the mineral is easily distinguished from 
the few other blue minerals known. 

Synthesis. — Crystals have been formed on calcite by allowing frag- 
ments of this mineral to lie in a solution of CUNO3 for a year or more. 

Occurrence. — The mineral occurs in the oxidized zone of copper veins. 
It is an intermediate product in the change of other copper compounds 
to malachite. 

Localities. — Azurite occurs in beautiful crystals at Cressy, France; 
near Redruth, in Cornwall; at Phoenixville, Penn.; at Mineral Point, 
Wis.; at the Copper Queen Mine, Bisbee, Ariz.; at the Mammoth 
Mine, Tintic district, Utah; at Hughes's Mine, California, and at many 
other copper mines in this country and abroad. 

From Morenci, Ariz., Mr. Kunz describes specimens consisting of 
spherical masses composed of alternating layers of malachite and 
azurite, which, when cut across, yield surfaces banded by alternations of 
bright and dark blue colors. 

Uses. — Azurite is mined with other copper minerals as an ore of cop- 
per. It is also used to a slight extent as an ornamental stone (see mal- 
achite). 

HYDROUS CARBONATES 

The hydrous carbonates are salts containing water of crystalliza- 
tion. They are carbonates of sodium or of this metal with calcium or 
magnesium. Some of them occur in abundance in the waters of salt or 
bitter lakes, but very few are known to occur in any large quantity in 
solid form. Among the commonest are: 

Soda or natron Na2C03 • 10H2O monoclinic 

Trona HNa3(C03)2-2H20 monoclinic 

Gaylussite Na2Ca(C03)2 • 5H2O monoclinic 

HydromagnesUe Mg4(OH)2(C03)3 • 3H2O orthorhombic 

These minerals occur either in the muds of lakes or as crusts upon the 
mud or upon other minerals. 





CARBONATES 235 

Natron occurs only in solution and in the dry mud on the borders 
of lakes. 

Trona, or urao, (HNa3(C0 3 )2 2H2O) is found as crystals in the 
mud of Borax Lake, California, as a massive bed in Churchill Co., 
Nevada, and as thin coatings on rocks in other 
places. Its crystallization is monoclinic (pris- 
matic class), with the axial ratio, 2.8426 : 1 : 
2.9494 and 0=76° 31'. Its crystals are usually 
bounded by oP(ooi), 00 Poo (ioo), — P(in) and Fig. 128.— Trona Crys- 
+P(Tn) (Fig. 128). Fibrous and massive forms tal with oP,ooi (c); 
are common. The mineral has a perfect cleavage °° p * ' IO ° ^ and 
parallel to 00 Pdb (100). It is gray or yellowish \ 11 

and has a colorless streak. It has a vitreous luster, a hardness of 
2.5-3, an d a density of 2.14. It is soluble in water and has an alkaline 
taste. It exhibits the usual reactions for Na and for carbonates. 

Gaylussite (Na2Ca(C03)2-5H20) also occurs as crystals in the 
muds of certain lakes, especially Soda Lake, near Ragtown, Nevada, 
and Merida Lake, Venezuela, and in clays under swamps in Railroad 

Valley, in Nevada. Its crystals are monoclinic 
(prismatic class) with alb: c— 1.4897 : 1 : 1.4442 
and 18=78° 27'. They are usually bounded by 
00 P(no), Po> (on), and £P(Ti2) (Fig. 129), or by 
these planes and oP(ooi) and 00 P 60 (100). They 
are either prismatic because of the predominance 
of Poo (on) and oP(ooi), or are octahedral in 
habit because of the nearly equal development of 
P 00 (on) and 00 P(no). Their cleavage is perfect 
Fig. 1 29.- Gaylussite parallel to ooP(no). 

t\ W ^ ' The mineral is white or yellowish and trans- 
no (w); Poo , on J 

(e) and iP, 112 (r). luc en t. Its hardness is 2-3 and density 1.99. 

It is very brittle. When heated in the closed 
tube it decrepitates and becomes opaque. It loses its water at ioo . 
In the flame it melts easily to a white enamel and colors the flame yellow. 
It is partially soluble in water, leaving a white powdery residue of CaCCfe 
and is entirely soluble in acids with effervescence. The mineral occurs 
in such large quantity in the clays underlying swamps in Railroad Valley, 
Nevada, that its use has been suggested as a source of NagCOs. 




CHAPTER XII 

THE SULPHATES 

The sulphates are salts of sulphuric acid. A large number are 
known to occur in nature but many of them are dissolved in the waters 
of salt lakes. Of the remaining ones only a few are very common. 
These may be divided into an anhydrous normal group, a basic group and 
a hydrated group. In addition, there are several minerals that are 
sulphates mixed with chlorides or carbonates. 

All the sulphates that are soluble in water give the test for sulphuric 
acid. When heated with soda on charcoal they are reduced to sulphides. 
The mass when placed on a silver coin and moistened with a drop of 
water or of hydrochloric acid partly dissolves and stains the silver dark 
brown or black. 

The sulphates when pure are all white and transparent, and are all 
nonconductors of electricity. 

ANHYDROUS SULPHATES 

NORMAL SULPHATES 

The anhydrous normal sulphates have the general formula R' 2 S04 
or R"S04. The most common ones are sulphates of the alkaline earths 
and lead. They belong in a single group which is orthorhombic. The 
few less common ones are sulphates of the alkalies or of the alkalies 
and alkaline earths. Only two of the latter are described. 

Glauberite (Na 2 Ca(S0 4 ) 2 ) 

Glauberite may be regarded as a double salt of the composition 
Na 2 S04 • CaS04, which requires 51.1 per cent Na 2 S04 and 48.9 per cent 
CaS04. The mineral contains 22.3 per cent Na 2 0, 20.1 per cent CaO 
and 57.6 per cent SO3. 

It nearly always occurs in monoclinic crystals (prismatic class), 
with an axial ratio 1.2209 : 1 : 1.0270 and P=6j° 49'. The most fre- 
quent combination is oP(ooi), — P(ni), ooP(no), ooPoo(ioo), 
3P3(3iT) and +P(nT), with oP(ooi) prominent (Fig. 130). The 
cleavage is perfect parallel to oP(ooi). The angle no A 110=96° 58'. 

236 



SULPHATES 



237 




Glauberite is yellow, gray or brick-red in color, is transparent or 
translucent and has a white streak, a vitreous luster and a conchoidal 
fracture. Its hardness is 2.5-3 and its specific 
gravity about 2.8. It is brittle. It is partly 
soluble in water, imparting to the solution a 
slight saltiness. The red color of many speci- 
mens is due to the presence of inclusions. 

Before the blowpipe the mineral decrepi- 
tates, whitens and fuses easily to a white 
enamel, at the same time coloring the flame Fig. 130— Glauberite Crys- 
yellow. It is soluble in HC1 and in a large tal with oP, 001, (c); °oP, 

i.\L £ t 11 4. 'a. e no (m); oo P oo , ioo (a) 

quantity of water. In a small quantity of h -P () 
water it is partially dissolved with loss of 
transparency and the production of a deposit of CaS04. 
It sometimes alters to calcite. 

Occurrence. — Glauberite is associated with rock salt and other de- 

posits from bodies of salt water. It is found 
at Villa Rubia, in Spain, and elsewhere 
in Europe, and in the Rio Verde Valley, 
Arizona and at Borax Lake, California. 




Fig. 131. — Thenardite Crystal 
with 00 P, no (w); P, 11T 
(0); iP«, 106 (/) and oP, 
oor(c). 



Thenardite (Na2S04) occurs as ortho- 
rhombic crystals in the vicinity of salt 
lakes, and in beds associated with other 
lake deposits. Its crystals have an axial ratio .5976 : 1 : 1.2524. 
They are commonly prismatic but those 
from California are tabular and are bounded 
by ooP(no), oP(ooi), P(iiT), JP *> (106), 
and 00 P* (100) (Fig. 131). Twins arc 
common (Fig. 132). 

The mineral is colorless, white or reddish 
and has a salty taste. Its hardness is 2-3 
and its specific gravity 2.68. Its inter- 
mediate refractive index is 1.470. It is 
readily soluble in water. It occurs in exten- 
sive deposits in the Rio Verde Valley, Ari- 
zona, and as crystals at Borax Lake, Cali- 
fornia and on the shores of salt lakes in 
Central Asia and South America. 




Fig. 132. — Thenardite 
Twinned about P 06 (on). 
Forms same as in Fig. 131 
and 00 P « , 100 (a). 



238 DESCRIPTIVE MINERALOGY 



BARITE GROUP 

The barite group includes the sulphates of the alkaline earths and 
lead. They are all light colored minerals with a nonmetallic luster. 
They all crystallize in the orthorhombic system (bipyramidal class), 
and all have a hardness of about 4. The minerals comprising this group, 
with their axial ratios, are: 

Anhydrite CaSC>4 alb: £=.8932 : 1 : 1.0008 

Barite BaSC>4 =.8152 : 1 : 1.3136 

Celestite SrSQi =.7790 : 1 : 1.2800 

AnglesUe PbS04 =-7852 : 1 : 1.2894 

Anhydrite (CaSOj 

Calcium sulphate is dimorphous. The natural compound, anhy- 
drite, is orthorhombic bipyramidal. In addition to this, there is another 
which passes over into anhydrite when shaken for a long time with boiling 
water. It is produced by dehydrating gypsum at about ioo°. When 
moistened it combines with water and passes back to gypsum. It is 
probably triclinic. It is unstable under the conditions prevailing at 
the earth's surface and is, therefore, not found as a mineral. 

Anhydrite occurs usually in fibrous, granular or massive forms, not 
often in crystals. When crystals occur they are commonly prismatic or 
tabular in habit. 

In composition the mineral is 58.8 per cent SO3 and 41.2 per cent 
CaO. 

Its crystals are usually bounded by the three pinacoids oP(ooi), 
00 Poo (100), 00 Poo (010) and P(ni), 2P2(i2i), 3P3(i3i), P*(ioi) 
and P06 (on). The prismatic types are usually elongated parallel to 
the macroaxis. The angle noAiTo=83° 41'. 

Anhydrite fuses quite easily before the blowpipe and colors the flame 
reddish yellow. It is very slightly soluble in water but is completely 
dissolved in strong sulphuric acid. It cleaves parallel to the three pin- 
acoids yielding rectangular fragments. Its hardness is 3-3.5 and den- 
sity about 2.95. Its luster is vitreous in massive pieces and its color 
white, often with a distinct tinge of blue, gray or red. In small frag- 
ments it is translucent, but in large masses it is opaque. Its refractive 
indices for yellow light are: a= 1.5693, 7= 1.6 130. 

It is distinguished from the other sulphates by its specific gravity 
and the color it imparts to the blowpipe flame. 



SULPHATES 239 

Synthesis. — Its crystals have been produced by slowly evaporating a 
solution of gypsum in H2SO4. 

Occurrence. — Anhydrite occurs as crystals implanted on the minerals 
of ore veins, as beds of granular masses associated with gypsum, and as 
crystalline masses in layers associated with rock salt — the two having 
been deposited by the evaporation of salt waters. 

Localities. — The mineral is found at the salt mines of Stassfurt, in 
Germany; Hall, in Tyrol; Bex, in Switzerland; in the ore veins of 
Andreasberg, in Harz; Bleiberg, in Carinthia, and at many other places 
in Europe. At Lockport, N. Y., and at Nashville, Tenn., it occurs as 
crystals lining geodes in limestone, and at the mouths of the Avon and 
St. Croix Rivers in Nova Scotia it forms large beds associated with 
gypsum. 

Uses. — Finely granular forms of the mineral are used for ornamental 
purposes, and as a medium for the use of sculptors. The massive variety 
is occasionally employed as a land plaster to enrich cultivated soils. 

Barite (BaS0 4 ) 

Barite, or heavy spar, usually occurs crystallized, though it is also 
often found massive and in granular, fibrous and lamellar forms. It is 
a common mineral associated with sulphide ores as a gangue. 

The mineral is sometimes pure but it is usually intermixed with the 
isomorphous calcium and strontium sulphates. The pure mineral con- 
tains 34.3 per cent SO3 and 65.7 per cent BaO. As usually mined it 
contains SiCfe, CaO, MgO, AI2O3, Fe2(h and in some instances PbS2 
(galena). 

The simple crystals are usually tabular or prismatic in habit. The 
tabular forms are commonly bounded by oP(ooi), ooP(no) and the 
domes, P 66 (101), JP w (102), 2P 66 (021), and P 66 (on), and sometimes 
P(ni) and 00 Poo (100) (Fig. 133). The prismatic forms are usually 
elongated in the direction of 
the a axis, and are bounded 
by the same planes as the 
tabular crystals (Fig. 134). FlG> I33 ._Barite Crystals with 00 P, no (m); 
Complex crystals are also JP5o, 102 (rf); P£,oii (0) and oP, 001 (c). 
abundant. They are often 

beautifully supplied with planes, the total number known on the 
species being about 100. The angle noAiTo=78° 225'. 

The cleavage of barite is perfect parallel to oP(ooi) and ooP(no). 
It is brittle. Its hardness is about 3 and its density about 4.5. The; 




^r^ 



240 DESCRIPTIVE MINERALOGY 

mineral is white, often with a tinge of yellow, brown, blue, or red. 

It is transparent or opaque and its streak is white. Its refractive 

indices for yellow light are: 0=1.6369, 7=1.6491. 

Before the blowpipe barite decrepitates and fuses, at the same time 

coloring the flame yel- 
lowish green. The fused 
mass reacts alkaline to 
litmus paper. It is in* 
soluble in acids. 

The mineral barite is 
Fig. i 3 4.-Barite Crystals with m, d t and c as in distinguished from the 

pT g 'x« 3 w Also °° P55 ' IO ° (fl); P ' IXI w " d other sul P hates b y its 

high specific gravity and 





the color it imparts to the blowpipe flame. 

Syntheses. — Crystals have been made by heating precipitated barium 
sulphate with dilute HC1 in a closed tube at 150°, and by cooling a fusion 
of the sulphate in the chlorides of the alkalies or alkaline earths. 

Occurrence and Origin. — Barite is a common vein-stone. It con- 
stitutes the gangue of many ore veins, particularly those of copper, 
lead and silver. It is found also as a replacement of limestone, which, 
when it weathers, leaves the barite in the form of fragments and nodules 
in a residual clay, and as a deposit in hot springs. In all cases it is 
believed to be a deposit from solutions. 

Localities. — Barite occurs abundantly in England, Scotland, and on 
the continent of Europe. Crystals are found at Cheshire, Conn.; at 
DeKalb, St. Lawrence Co., N. Y.j at the Phoenix Mine in Cabarrus 
Co., N. C, and near Fort Wallace, New Mexico. Massive barite in 
pieces large enough to warrant polishing is found on the bank of 
Lake Ontario, at Sacketts Harbor, N. Y. It constitutes the filling of 
veins at many different places, more particularly in the southern Appa- 
lachians and in the Lake Superior region. 

Preparation. — Much of the mineral that enters the trade in the 
United States is obtained from the deposits in residual clay. The rough 
material is washed, hand picked, crushed, ground and treated with 
sulphuric acid. The acid dissolves most of the impurities and leaves 
•the powdered mineral white. 

Uses. — The white varieties of the mineral are ground and the powder 
is used in making paints. The mineral is also employed in the manu- 
facture of paper, oilcloth, enameled ware, and in the manufacture of 
barium salts, the most important of which is the hydroxide, which is 
employed in refining sugar. 



SULPHATES 



241 



The colored massive varieties, more especially stalactitic and fibrous 
forms, are sawn into slabs, polished and used as ornamental stones. 

Production. — The quantity of barite mined in the United States 
during 1912 was over 37,000 tons, valued at $153,000. The principal 
producing states are Missouri, Tennessee and Virginia. The imports 
in the same year were about 26,000 tons of crude material, valued at 
$52,467 and 3,679 tons of manufactured material, valued at $26,848. 
Besides, there were imported $70,300 worth of artificial barium sul- 
phate and about $280,000 worth of other barium salts, exclusive of 
witherite. 

Celestite (SrS0 4 ) 

Celestite occurs in tabular prismatic crystals, in fibrous and some- 
times in globular masses. Though usually white, it often possesses a 
bluish tinge, to which it owes its name. 

The theoretical composition of the mineral is 43.6 per cent SO3 
and 56.4 per cent SrO, but it often contains small quantities of the 
isomorphous Ca and Ba compounds. 

Many celestite crystals are very similar in habit to those of barite. 





Fig. 135. — Celestite Crystals with <*> P, no (m); JP«o, 102 (d); J Poo, 104 (r); 

00 P 00 , 010 (6); P 00 , on (0) and oP, 001 (c). 



Tabular forms are perhaps more common (Figs. 135). Occasionally, 
pyramidal crystals are bounded by P4(i44), 00 Poo (100), Po6(on) 
and oP(ooi). These often have rounded edges and curved faces and 
thus come to have a lenticular shape. The angle no A iTo= 75 ° 50'. 

The cleavage of the mineral is perfect parallel to oP(ooi) and almost 
perfect parallel to ooP(no). Its hardness is about 3 and its specific 
gravity 3.95. Its luster and streak are like those of barite. Its color 
is often pale blue and sometimes light red, but pure specimens are 
white or colorless. Its refractive indices for yellow light are: a= 1.6220, 
7=1.6237. 

Before the blowpipe celestite reacts like barite except that it tinges 
the flame crimson. This crimson color may be obtained more dis- 
tinctly by fusing a little powder of the mineral on charcoal in the reduc- 



242 DESCRIPTIVE MINERALOGY 

ing flame and dissolving the resulting mass in a small quantity of hydro- 
chloric acid, then adding some alcohol and igniting the mixture. 

Syntheses. — Crystals of celestite are produced in ways analogous 
to those in which barite crystals are formed. 

Occurrence and Origin. — Celestite occurs in beds with rock salt and 
gypsum, as at Bex, Switzerland; associated with sulphur, as at Gir- 
genti, Italy; and in crystals and grains scattered through limestone, 
as at Strontian Island, Lake Erie, and in Mineral Co., W. Va., or 
as crystals lining geodes in the same rock. It is also sometimes found 
as a gangue in mineral veins. In some instances it was deposited by 
hot waters, in others by cold waters, and in others it was concentrated 
by the leaching of strontium-bearing limestones by atmospheric water. 

Production and Uses. — Although the mineral occurs in large quan- 
tity at a number of places in the United States and Canada it is not 
mined. A small quantity of the strontium oxide is annually imported. 
Strontium salts, prepared from celestite in part, are used in the manu- 
facture of fireworks and medicines and in refining sugar. 

Anglesite (PbS0 4 ) 

Anglesite occurs principally as crystals associated with galena and 
other ores of lead, but is found also massive, and in granular, stalactitic 
and nodular forms. 

The theoretical composition of the mineral demands 73.6 per cent 
PbO and 26.4 SO3. 

Its orthorhombic crystals are usually prismatic or isometric in habit. 
Tabular habits are less common than in barite and celestite. The 
principal forms occurring are ooP<x> (100), o°P(no), £P * (102), and 
other macrodomes, P 06 (on) and various small pyramids, with oP(ooi), 
in addition, on the tabular crystals (Figs. 156, 137, 138). The angle 
no A iTo=76° i6|'. 

The cleavage of anglesite is distinct parallel to oP(ooi) and 00 P(i 10). 
Its fracture is conchoidal. The mineral is white, gray or colorless and 
transparent, and is often tarnished with a gray coating. It has an 
adamantine or residuous luster, is brittle and has a colorless streak. 
Its hardness is 2/5-3 and sp. gr. 6.3-6.4. Impure varieties may be 
tinged with yellow, green or blue shades and in some cases may be 
opaque. Its refractive indices for yellow light are : a = 1 .877 1 , 7 = 1 .893 7. 

Before the blowpipe anglesite decrepitates. It fuses in the flame of 
a candle. On charcoal it effervesces when heated with the reducing 
flame and yields a button of metallic lead. In the oxidizing flame it 



SULPHATES 



243 



gives the lead sublimate. The mineral dissolves in HNO3 with dif- 
ficulty. 

The mineral is characterized by its high specific gravity and the 




m 



m 





Fig. 136.- 
Fig. 137.- 



Fig. 136. Fig. 137. 

-Anglesite Crystal with <*> P, no (m); 00 P qo , xoo (a); oP, 001 (c); 

}P, 112 (r) and PX 122 (y). 

-Anglesite Crystal with m, a and y as in Fig. 136. Also 00 Poo, 
cio l&}, P 00 , 011 (0); P, in (s) and JP 00 , 102 (d). 




reaction for lead. It is distinguished from cerussite by the reaction for 
sulphur and the lack of effervescence with HCl. 

Syntheses. — Crystals of anglesite have been made by methods anal- 
ogous to those used in the preparation of barite crystals. 

Occurrence. — The mineral occurs as an alteration product of galena, 
mainly in the upper portions of veins of 
lead ores. Under the influence of solu- 
tions of carbonates it changes to cerus- 
site. 

Localities. — It is found in Derby- 
shire and Cumberland, in England; 
near Siegen, in Prussia; in Australia and 
in the Sierra Mojada, in Mexico. In the 
United States crystals occur at Phoenix- 

ville, Penn., in the lead districts of the Mississippi Valley, and at 
various points in the Rocky Mountains. 

Uses. — It is mined with other lead compounds as an ore of this metal. 

BASIC SULPHATES 

Although several basic sulphates are known as minerals, only two 
are of importance. One, brochantite, is a copper compound found, with 
other copper minerals, in the oxidized portions of ore veins, and the 
other, alunite, is a double salt of aluminium and potassium. This min- 



Fig. 138. — Anglesite Crystal with 
m, y, c and d as in Figs. 136 and 
137. Also JP 00 , 104 (/) and P4, 

144 (x). 



244 DESCRIPTIVE MINERALOGY 

eral is one of a series of compounds forming an isomorphous group, with 
the general formula (R ,/, (OH) 2 )6R / 2(S0 4 )4 or (R'"(OH) 2 ) 6 R"(S04)4, 
in which R'" = A1 or Fe, R' 2 = K 2 , Na 2 or H 2 and R"=Pb. 

Alunite ((A1(OH)2)gK 2 (S0 4 ) 4 ) 

Alunite, or alumstone, is a comparatively rare mineral, but, because 
of its possible utilization as a source of potash, it is of considerable in- 
terest. It has long been used abroad as a source of potash alum. 

The mineral, when pure, contains 38.6 per cent SO3, 37.0 per cent 
A1 2 C>3, 1 1. 4 per cent K 2 and 13.0 per cent H 2 0, which corresponds to 
the formula given above, or if written in the form of a double salt 
3(Al(OH) 2 ) 2 S04-K 2 S04. The chemical composition of a crystalline 
specimen from Marysville, Utah, is as follows: 

SO3 A1 2 3 Fe 2 3 P2O5 K 2 Na 2 H 2 0+ H 2 0- Si0 2 Total 
38.34 37- x 8 tr. .58 10.46 .33 12.90 .09 .22 100.10 

Alunite occurs in hexagonal crystals (di trigonal scalenohedral class), 
with an axial ratio of 1 : 1.252. The natural crystals are nearly always 
simple rhombohedrons, R(ioTi), or R modified by other rhombohedrons 
and the basal plane. Because the angle between the rhombohedral 
faces is about 90 (90 50'), the habit of the crystals is cubical. The . 
mineral also occurs massive, with fibrous, granular or porcelain-like 
structure. 

Alunite is white, pink, gray or red, and has a white streak. It is 
transparent or translucent and has a vitreous or nearly pearly luster. 
Its cleavage is distinct parallel to oP(oooi), and it has an uneven, con- 
choidal or earthy fracture. Its hardness is 3.5-4 and its density = 
2.6-2.75. Its indices of refraction for yellow light are: 6=1.592, 
(0=1.572. 

Before the blowpipe the mineral decrepitates, but is infusible. In 
the closed tube it yields water and at a high temperature sulphurous and 
sulphuric oxides. Heated on charcoal with Co(N03) 2 it gives the blue 
color characteristic of AI22O3. It also gives the sulphur reaction. It is 
insoluble in water but is soluble in H2SO4. When ignited it gives off 
all its water and three-quarters of its SO4, the other quarter remaining 
in K2SO4. When the ignited residue is treated with water, the potas- 
sium sulphate dissolves and insoluble AI2O3 is left. It is upon this 
latter reaction that the economic utilization of the mineral depends. 

The mineral is characterized by its color and hardness together 
with the reactions for A1,H 2 and sulphuric acid. 



SULPHATES 245 

Synthesis. — Crystals have been made by heating an excess of alu- 
minium sulphate with alum and water at 230 . 

Occurrence and Origin. — The mineral occurs in seams or veins in 
acid lavas. It is thought to have been formed in some instances by 
the action of sulphurous vapors upon the rock forming the vein walls, 
in other instances by direct precipitation from ascending magmatic 
waters, and in others by the action of descending H2SO4. 

Localities. — The principal known occurrences of alunite are at 
Tolfa, Italy; at Bulla Delah, New South Wales; on Milo, Grecian 
Archipelago, and at Mt. Dore, France. 

In the United States it is found with quartz and kaolin in the 
Rosita Hills, and the Rico Mts., Colo.; in the ore veins at Silverton 
and Cripple Creek, Colo.; as a soft white kaolin-like material in the 
ore veins at Goldfield, Nev.; as a crystalline constituent in the rocks 
at Goldfield, Nev., and Tres Cerritos, Cal., and in the form of a great 
vein of comparatively pure material at Marysville, Utah. 

Uses. — In Australia alunite is calcined and then heated with dilute 
sulphuric acid. The mixture is then allowed to settle and the clear 
solution is drawn off and cooled. Alum crystallizes. The mother liquor 
which contains aluminium sulphate, after further treatment with the 
calcined mineral, is evaporated and the aluminium salt separated by 
crystallization. In the United States it is now (1916) being utilized 
as a source of potash and aluminium. 

Brochantite ((CuOH) 2 S0 4 2Cu(OH) 2 ) occurs in groups of small 
prismatic crystals, in fibrous masses and in drusy crusts. Its crystal- 
lization is orthorhombic with a : b : c=.yy^g : 1 : .4871 and the angle 
iioAiTo=75° 28'. Cleavage is perfect parallel to 00 P 06 (010). The 
mineral is emerald-green to blackish green and its streak is light 
green. It is transparent or translucent, and its luster is vitreous, 
except on cleavage planes where it is slightly pearly. Its hardness is 
3.5-4 and density 3.85. In the closed tube it decomposes, yielding 
water and, at a high temperature, sulphuric acid. It gives the usual 
reactions for copper and sulphuric acid. Brochantite occurs in the 
upper portions of copper veins at many places, in some of which it was 
formed by the interaction between silicates and solutions of copper 
salts. In the United States it has been found at the Monarch Mine, 
Chaffee Co., Colorado, at the Mammoth Mine, Tintic District, Utah, 
and in the Clifton-Morenci Mines, Arizona, 



246 DESCRIPTIVE MINERALOGY 

HYDROUS SULPHATES 

The hydrous sulphates comprise a number of sulphates combined 
with water. Among them are the normal salts mirabUite or glauber 
salt (Na2S04- 10H2O), gypsum (CaSO-r 2H2O), the epsomite and melan- 
terite groups (R"S0 4 -7H 2 0), chalcanthite (CUSO45H2O), and the 
alum group (R'A1(S04)2- 12H2O), kieserite (MgS04*H20), polyhalite 
(K.2MgCa2(S04)4-H20), and a number of basic compounds. Several 
of them are of considerable economic importance. They are separated 
into a normal group and a basic group, 

HYDRATED NORMAL SULPHATES 

The hydrated normal sulphates occur in crystals, and most of them 
are found also in beds interstratified with other compounds that are 
known to have been precipitated by the evaporation of sea water or the 
water of salt and bitter lakes. All are soluble in water. 

Mirabilite, or glauber salt, (Na 2 S0 4 • 10H2O) is a white, trans- 
parent to opaque substance occurring in monoclinic crystals or as 
efflorescent crusts. Its hardness is 1.5-2 and specific gravity 1.48. It 
is soluble in water and has a cooling taste. When exposed to the air it 
loses water and crumbles to a powder. Mirabilite occurs at the hot 
springs at Karlsbad, Bohemia and is obtained from the water of many 
of the bitter lakes in California and Nevada. Its crystals are deposited 
from a pure solution of Na2S04. If the solution contains NaCl, how- 
ever, thenardite (Na2S04) deposits. 

Kieserite (MgS04 • H2O) occurs commonly in granular to compact, 
massive beds interstratified with halite and other soluble salts at Stass- 
furt, Germany, and at other places where ocean water has been evap- 
orated. It is believed to have resulted from the partial desiccation of 
epsomite (MgS04*7H20), though it may be deposited from a solution 
of MgS04 in the presence of MgCb. Kieserite is white, gray, or yellow- 
ish, and is transparent or translucent. It forms sharp bipyramidal 
monoclinic crystals. Its hardness is 3 and its density 2.57. In the 
presence of water it passes over into epsomite and dissolves, yielding a 
solution with a bitter taste. Large quantities are utilized in the fer- 
tilizer industry. 

When exposed to the air it becomes covered with an opaque crust. 



SULPHATES 



Gypsum (CaS0 4 aH-O) 

Gypsum is the most important of all the hydrous sulphates. It 
occurs in massive beds as:sociated with limestone, in crystals, in finely 
granular aggregates and in fibrous masses, under a great variety of 
conditions. 

Theoretically, it consists of 46.6 per cent SO3, 32.5 per cent CaO and 
30.9 per cent H2O, but usually it contains also notable quantities of other 
components, especially Fe203, AI2O3 and Si02- Clay is a common im- 
purity in the massive varieties. 

The analyses of two commercial gypsums follow: 



CaSO., 

Dillon, Kans 78 . 40 

Alabaster, Mich 78.51 



H 2 Si02 AI2O3 CaC03 MgC0 3 Total 
10.06 .35 .12 .56 .57 99.96 
2096 .05 .08 ... .11 9971 



The crystals are monoclinic {prismatic class), with a : b : £=.6895 : 
1 : .4132 and (J=8i° 02'. They are usually developed with a tabular 
habit due to the predominance of °oPm>(oio). The prism «>P(iio), 



Fie. 139. 
Fio. 139. — Gypsum Crystals with ' 



w 

Fio. 140. 
a (ft); -P, in (0 and 



o(«i); =»P 
JP 5>, T03 W. 
FlC. i40.r~<iypmni Twinned about »P» (1°°). Swallow-tail Twin. Form tn, 
/ and b as in Fig. J3y. 



and pyramid +P(irT) are also nearly always present {Fig. 139). Often 
the +P faces are curved, producing a lens-shaped body. Twinning is 
very common, giving rise to two types of twinned crystals. In the most 
common of these 00 P 00 {too) is the twinning plane and the resulting 
twin has the form of Fig. 140. In the second type — Poo (101) is the 
twinning plane {Fig. 141). Forms of this type are frequently bounded 
by +PC11T), -P(iii), JP*(7oj), and 00 P do (100). When the side 



248 



DESCRIPTIVE MINERALOGY 





faces are curved the well known arrowhead twins result (Fig. 141). 

The angle noAiTo=68° 30'. 

The mineral possesses a good cleavage parallel to 00 P 00 (010) 

yielding thin inelastic foliae, another parallel to +P(Tn) and a less 

perfect one parallel to 00 P * (100). 
It is white, colorless and transpar- 
ent when pure; gray, red, yellow, 
blue or black when impure. Its 
hardness is 1.5-2 and sp. gr.= 2.32. 
The luster of crystals is pearly on 
00 P ob (010) and on other surfaces 
vitreous. Massive varieties are often 
dull. The refractive indices for yel- 
low light are: a= 1.5205, 0= 1.5226, 

Fig. 141. — Gypsum Twinned about '" '^ ' 

-P «(ioi). Forms: °° P ao , 100 I n tne closed tube the mineral 
(a); -P, in (/); P, 11T («) and gives off water and falls into a white 
JPq6,To3(*). Arrow head Twin, powder (see p. 238). It colors the 

flame yellowish red and yields the sul- 
phur test on a silver coin. It is soluble in about 450 pts. of water and 
is readily soluble in HC1. When heated to between 222 F. and 400 F. 
it loses water and disintegrates into powder, which, when ground, 
becomes " plaster of Paris." This, when moistened with water, again 
combines with it and forms gypsum. The crystallization of the mass 
into an aggregate of interlocking crystals constitutes the " set." 

Gypsum is distinguished from other easily cleavable, colorless min- 
erals by its softness and the reactions for S and H2O. 

The varieties of gypsum generally recognized are: 

Selenite, the transparent crystallized variety; 

Satins par, a finely fibrous variety; 

Alabaster, a fine-grained granular variety, and 

Rock-gypsum, a massive, structureless, often impure and colored 
variety. 

Gypsite is gypsum mixed with earth. 

Syntheses. — Crystals of gypsum separate from aqueous solutions of 
CaS04 at ordinary temperatures, and also from solutions saturated 
with NaCl and MgCb. Some of these are twinned. 

Occurrence and Origin. — Gypsum forms immense beds interstrati- 
fied with limestone, clay and salt deposits where it has been precipitated 
by the evaporation of salt lakes. Its crystals occur around volcanic 
vents, where they are produced by the action of sulphuric acid on cal- 



SULPHATES 249 

careous rocks. They are also found isolated in day and sand, and in 
limestone, wherever this rock has been acted upon by the sulphuric acid 
resulting from the weathering of pyrite. Gypsum also occurs in veins 
and is found in New Mexico in the form of hills of wind-blown sand. 

Localities. — Crystals are found in the salt beds at Bex, Switzerland; 
in the sulphur mines at Girgenti, Sicily, and at Montmartre, France. 
In the United States they occur at Lockport, N. Y., in Trumbull Co., 
Ohio, and in Wayne Co., Utah, in limestone; and on the St. Mary's 
River, Maryland, in clay. 

Extensive beds occur in Iowa, Michigan, New York, Virginia, Ten- 
nessee, Oklahoma and smaller deposits in many other states, and wind- 
blown sands in Otero Co., New Mexico. 

Uses. — Crude gypsum is used in the manufacture of plaster, as a 
retarder in Portland cement, and as a fertilizer under the name of land 
plaster. The calcined mineral is used as plaster of Paris and in the 
manufacture of various wall finishing plasters, and certain kinds of 
cements. Small quantities are used in glass factories, and as a white- 
wash, a deodorizer, to weight phosphatic fertilizer, as an adulterant in 
candy and other foods, and as a medium for sculpture. 

Production. — The quantity of gypsum mined in the United States 
during 1912 aggregated 2,500,757 tons, valued at $6,563,908 in the form 
in which it was sold. Of this amount, 441,600 tons of crude material, 
valued at $623,500 were sold ground, and 1,731,674 tons, valued at $5,- 
940,409, were calcined. The output of New York was valued at $1,241,- 
500, that of Iowa at $845,600 and of Ohio at $812,400. 

After the United States the next largest producer is France with a 
product in 1910 of 1,760,900 tons, valued at $2,942,600 and Canada with 
525,246 tons, valued at $934,446. 

EPSOMITE AND VITRIOL GROUPS 

These groups comprise minerals with the general formula RSO4 • 7H2O, 
in which R=Mg, Zn, Fe, Ni, Co, Mn and Cu. Isomorphous mix- 
tures indicate that the compounds are diomorphous, and that the 
group is, therefore, an isodimorphous group. The group is separable 
into two divisions, of which one, the epsomite group, crystallizes in the 
bisphenoidal class of the orthorhombic system with axial ratios approx- 
imating 1:1: .565. The other division, the vitriol, or melanterite, 
group crystallizes in the prismatic class of the monoclinic system with 
axial ratios approximating 1.18 : 1 : 1.53 and approximating 75 . 
Only the magnesium compound among the pure salts is known to crys- 
tallize in both systems. Crystals separated from a saturated solution 




m 



250 DESCRIPTIVE MINERALOGY 

are orthorhombic, while those separated from a supersaturated solution 
are monoclinic. Other salts occur in isomorphous mixtures in both 
systems. All members of the group are soluble in water and all occur as 
secondary products formed by decomposition of other minerals. 

Epsomite (MgS0 4 7H2O) 

Epsomite, or Epsom salt, usually occurs in botryoidal masses and 
fibrous crusts coating various rocks over which dilute magnesium sul- 
phate solutions trickle, and mingled with earth 
in the soils of caves. The solutions result from 
the action upon magnesian rocks of sulphuric 
acid derived from oxidizing sulphides. Crys- 
tals are rare. 

The composition corresponding to MgS04 • 
I i 7H2O demands 32.5 SO3, 16.3 MgO and 51.2 

^7 H2 °- 

The mineral forms white or colorless bi- 
Fig. i 4 2.~Epsomite Crys- sp h en oidal, orthorhombic crystals, with an 

tal with 00P, no (m) . . . ... . «,. . 

p axial ratio alb: c=.oooi : 1 : .5709. Their 

and -r, in (s). habit is tetragonal. The angle no A 110=89° 

26'. The commonest forms occurring on syn- 

P P 

thetic crystals are combinations of ooP(no), and — r(in) or —/(in) 

2 2 

(Fig. 142). Natural crystals contain, in addition 00 Poo (010) and 

P 06(101). 

The luster of epsomite is vitreous, its hardness 2.0-2.5 and specific 
gravity 1.70. Its refractive indices for yellow light are: a= 1.4325, 
0= 1.4554 and 7= 1.4608. 

The mineral is soluble in water, yielding a solution with a bitter taste. 
With a solution of barium chloride it yields a white precipitate of BaS04. 

Epsomite is distinguished from other colorless, soluble minerals by 
its taste and the reactions for S and Mg. 

Synthesis. — Crystals are produced by evaporation of solutions of 
MgS04 containing certain other salts. From those containing borax, 
crystals of the type indicated above are separated. The production of 
right or left crystals may be provoked by inoculation of the solution with 
a particle of a crystal of the desired type. 

Localities. — Epsomite occurs in mineral waters, as, for instance, at 
Seidlitz, Bohemia, on the walls of mines and caves, among the deposits 
of bitter lakes, and as crystals in the soil covering the floors of caves. 



SULPHATES 251 

Melanterite, or copperas (FeS0 4 7H2O), is usually in fibrous, 
stalactitic or pulverulent masses associated with pyrite or other sul- 
phides containing iron, from which it was produced by weathering 
processes. It is commonly some shade of green. Its streak is colorless. 
Its crystals, which are monoclinic (prismatic class), are rare. The 
mineral has a hardness of 2 and a density of 1.9. It is soluble in water, 
forming a solution which has a sweetish astringent taste. 

ALUM GROUP 

The alum group includes a large number of isomorphous compounds 
with the general formula R'Al(SC>4)2-i2H20. The group crystallizes 
in the isometric system (dyakisdodecahedral class), but all of its mem- 
bers are so readily soluble in water that they are rarely found in nature. 
The commonest alums are kalinite (KAl(S04)2-i2H20) and soda alum 
(NaAl(S0 4 ) 2 -i2H 2 0). 

DOUBLE SULPHATES WITH CARBONATES OR CHLORIDES 

A number of compounds of sulphates with chlorides and carbonates 
are known, but of these only one is of any great economic importance. 
Two others afford interesting crystals. The commercial compound is 
kainite, which is a hydrated combination of MgS04 and KC1, with 
the formula MgS04 • KG • 3H2O. The other two best known members 
of the group are leadhillite (PbS04-Pb(PbOH) 2 (C0 3 )2 and hanksite 
(2Na 2 C0 3 • oNa 2 S0 4 • KC1). 

Kainite (MgS0 4 KC1 3 H 2 0) 

Kainite is found only in beds associated with halite and other deposits 
from saline waters. It is rarely crystallized. Crystals are monoclinic 
(prismatic class), with a:6:c=i.2i86:i: .5863 and 0=85° 6'. They 
possess a pyramidal habit with oP(ooi) and ±P(ni)(iiT) predom- 
inating. 

The mineral usually forms granular masses which are white, yellow, 
gray or red. It is transparent, has a hardness of 2 and sp. gr. 2.13, 
and is easily soluble in water. Its refractive indices for sodium light are: 
a= 1.4948 and 7 = 1.5203. 

When heated in a glass tube it yields water and HC1. It is distin- 
guished from other soluble minerals by this reaction, and by the fact 
that it yields the test for sulphur, and colors the flame blue when its 
powder is mixed with CuO and heated before the blowpipe. 



252 DESCRIPTIVE MINERALOGY 

Synthesis. — Crystals have been produced by evaporating a solution 
of K2SO4 and MgSC>4 containing a great excess of MgCi2. 

Occurrence. — Kainite occurs in the salt beds of Stassfurt, Germany, 
and of Kalusz in Galicia, and in the deposits of salt lakes and lagoons. 
It also occurs as crusts on some of the lavas of Vesuvius. 

Uses. — The mineral is utilized as a source of potassium in the manu- 
facture of potassium salts and fertilizers. Large quantities are imported 
annually into the United States. In 191 2 the imports aggregated 
485,132 tons, valued at $2,399,761. 




Hanksite (2Na2C0 3 9Na 2 S0 4 KC1) occurs almost exclusively in 

hexagonal prisms that are prismatic or tabular, 
or in double pyramids suggesting quartz crys- 
tals. Their axial ratio is 1 : 1.006. The com- 
monest crystals are bounded by oP(oooi), 
Fig. 143.— Hanksite Crys- ooP(ioTo), P(ioTi) (Fig. 143) and 2P(202i), 
tal with cop, 1010 (m); or ^(4045). Their cleavage is imperfect 

P, ion (0) and oP, 0001 hi- t>/ \ <™. • 1 • v-^ 

/ v parallel to oP(oooi). The mineral is white or 

yellow. Its hardness =2 and its specific 

gravity =2.56. It is soluble in water. Its refractive indices are: 

o)= 1.4807 and €=1.4614. It occurs at Borax Lake and Death valley, 

California, in the deposits of salt lakes. 



Leadhillite (PbS0 4 Pb(PbOH) 2 (C0 3 )2) occurs principally as 
crystals in the oxidized zones of lead and silver veins. The crys- 
tals are monoclinic (prismatic class), and have an hexagonal habit. 
Their axial ratio is 1.7515 : 1 : 2.2261. /3=89°32'. The principal 
forms observed on them are oP(ooi), ooP(no), 00 Poo (100), P(m) 
and §P6o (102). In the most common twins ooP(no) is the twin- 
ning plane. The mineral is white or yellow, green or gray, and it is 
transparent or translucent. Its streak is colorless. It is sectile, has a 
hardness of 2.5 and a specific gravity of 6.35. Before the blowpipe it 
intumesces, turns yellow, and fuses easily (1.5). Upon cooling it again 
becomes white. It effervesces in HNO3 and leaves a white precipitate 
of PbS04. It reacts for sulphur and water. It is found at Leadhills, 
Scotland, and Mattock, England, associated with other ores of lead; 
at a lead mine near Iglesias, Sardinia, and at several silver-lead mines 
in Arizona. 



CHAPTER XIII 



THE CHROMATES, TUNGSTATES AND MOLYBDATES 



THE CHROMATES 

The only chromate of importance, among minerals, is the lead salt of 
normal chromic acid, H2C1O4. There are several other chromates 
known, but they are basic salts and are rare. All are lead compounds. 
The normal salt, PbCrCU, is known as crocoite. Chromic acid is un- 
known, as it spontaneously breaks down into C1O3 and water when set 
free from its salts. Its best known compound is potassium chromate, 
K^CrCU. 

Crocoite (PbCr0 4 ) 

Crocoite is well characterized by its hyacinth-red color. It is a lead 
chromate with PbO=68.g per cent and 003 = 31.1 per cent. 

Its crystallization is monoclinic 
(prismatic class) with a : b : c 
= .9603 : 1 : .9159 and £=77° 33'. 
Its crystals, which are usually im- 
planted on the walls of cracks in 
rocks, are prismatic or columnar 
parallel to ooP(no). Their pre- 
dominant forms are oop(no), 
— P(iii), and various domes (Fig. 
144). Their cleavage is distinct 
parallel to ooP(no). The angle 
iioAiio=86° 19'. The mineral 
also occurs in granular masses. 

Crocoite is bright hyacinth-red, 
and is translucent. Its streak is 
orange-yellow. The mineral is sec- 
tile. Its fracture is conchoidal, its 
hardness 2.5-3 an d density about 6. 





Fig. 144. — Crocoite Crystals with 00 P f 
no (m); 00 P2, 120 (/); -P, in (/); 
3P*. 301 (*); P«, 101 (*); oP, 
001 (c); Pob, on («); 2Pob f o2i (y) 
and iPob , 012 (u>). 

Its intermediate refractive index 



is about 2.42. 

In the closed tube it decrepitates, and blackens; but it reassumes its 
red color when heated. On charcoal it deflagrates and fuses easily, 

253 



254 DESCRIPTIVE MINERALOGY 

yielding metallic lead and a lead coating. With microcosmic salt it 
gives the green bead of chromium. 

The mineral is easily recognized by its color and the test for chro- 
mium. 

Synthesis. — Crystals, like those of crocoite, have been obtained by 
heating on the water bath a solution of lead nitrate in nitric acid and 
adding a dilute solution of potassium bichromate. ' 

Occurrence. — Crocoite occurs under conditions which suggest that it 
is a product of pneumatolysis. 

Localities. — It is found in the Urals; at Rezbanya and Moldawa, in 
Hungary; in Tasmania, and in the Vulture Mining district, Maricopa 
Co., Arizona. 

THE TUNGSTATES AND MOLYBDATES 

The tungstates are salts of tungstic acid, H2WO4. They are the 
principal sources of the metal tungsten which is beginning to have im- 
portant uses. The molybdates are salts of moiybdic acid, H2M0O4. 
The two most prominent tungstates are scheelite, CaWCk, and wolf- 
ramite (Fe-Mn)W04, and the most prominent molybdate is wulfenite, 

PbMo0 4 . 

All tungsten compounds give a blue bead with salt of phosphorus in 
the reducing flame. When fused with Na2C(>3, dissolved in water 
and hydrochloric acid, and treated with metallic zinc (see pp. 482, and 
492 for details of test), they also yield a blue solution which rapidly 
changes to brown. 

The molybdates give with the salt of phosphorus bead in the oxidiz- 
ing flame a yellow-green color while hot, changing to colorless when cold. 
In the reducing flame the color is clear green. 

SCHEELITE GROUP 

The scheelite group comprises a series of tungstates and molybdates 
of Ca, Cu and Pb. The minerals are tetragonal and hemihedral and 
are all well crystallized. The more important members of the group 
are scheelite and wulfenite. Cuprotungstite is a copper tungstate (CuWCU) 
and stolzite a lead tungstate (PbW04). 

Scheelite (CaW0 4 ) 

The formula of scheelite demands 80.6 per cent WO3, and 19.4 per 
cent CaO, but the mineral usually contains a little molybdenum in 
place of some of the tungsten. It nearly always contains also a little Fe. 



CHROMATES, TUNGSTATES AND MOLYBDATES 255 

Scheelite crystallizes in the tetragonal bipyramidal class. Its crys- 
tals are usually pyramidal, though often tabular in habit. Their axia! 
ratio is i : 1.5268. On the pyramidal types the predominant planes 
are pyramids of the first, second (Fig. 145), and third orders and on the 
tabular types, in addition, the basal plane. One of the most familiar 

combinationsisPtxxO.Poo (loI ). ^(3.3) and [^]( 13 (Fig. 145). 

Other forms frequently found on its crystals are £P oo (102) and £P °° 
(105). The angle iioaTii = 79° 55 J'.. Twinning is common, both 
contact and penetration twins having °o P 00 (100) as the twinning 
plane. The mineral also occurs in reniform and granular masses. 

Scheelite is white, yellow, brown, greenish or reddish, with a white 





Fig. 145. Fig. 146. 

Fig. 145. — Scheelite Crystal with P, n 1 \p); Poo , 101 (e) and oP, 001 (c). 

Fig. 146. — Scheelite Crystal with P and e as in Fig. 145. Also I 1 , 313 (h) and 

f3 p 3l , n L 2 J 

L"TJ» r 3 x W- 

• 

streak and vitreous luster. It has a distinct cleavage parallel to P(ooi), 
and an uneven fracture. It is brittle, has a hardness of 4.5-5 an( i a 
density of about 6, and is transparent or translucent. It is soluble in 
HC1 and HNO3 with the production of a yellow powder, tungsten tri- 
oxide, which is soluble in ammonia. Its refractive indices are: e= 1.9345, 
«= 1.9185 for red light. 

Before the blowpipe the mineral fuses to a semitransparent 
glass. With borax it forms a transparent glass which becomes opaque 
on cooling. With salt of phosphorus it yields the characteristic beads 
for tungsten, but specimens containing iron must be heated with tin on 
charcoal before the blue color can be developed. 

Scheelite is distinguished from limestone, which its massive forms 
closely resemble, by its higher specific gravity and the absence of effer- 



266 DESCRIPTIVE MINERALOGY 

vescence with HC1. From quartz it is distinguished by its softness and 
from barite by greater hardness and higher specific gravity. 

Syntheses. — Crystals of scheelite have been made by adding a solu- 
tion of sodium tungstate to a hot acid solution of CaCk, and by fusing 
the two compounds. They have also been produced by fusing wolfram- 
ite with CaCl 2 . 

Occurrence and Origin. — Scheelite is found in gold-quartz veins 
and in veins cutting acid igneous rocks, where it is associated with 
cassiterite, topaz, fluorite, molybdenite, wolframite and many other 
metallic compounds, and as a contact metamorphic product in altered 
limestone intruded by granite. It is probably in all cases a deposit 
from hot solutions. 

Localities. — It occurs at Zinnwald, Bohemia; Altenberg, Saxony; 
Carrock Fells, Cumberland, England; Pitkaranta, Finland; in New 
Zealand; and in the United States at Monroe and Trumbull, Conn.; in 
the Atolia District, Kern Co., California; the Mammoth Mining Dis- 
trict, Nevada; in Lake County, Colorado; near Gage, New Mexico, 
where it occurs with pyrite and galena in a vein cutting limestone, 
and in the placer gravels at Nome, Alaska. 

Uses of Tungsten. — Tungsten is used principally in the manufacture 
of tool steel, electric furnaces and targets for Rontgen rays. It is 
employed also as filaments in electric-light bulbs, in the manufacture 
of sodium tungstate which is used for fireproofing cloth, as a mordant 
in dyeing, and for a number of other minor purposes. 

Production. — Scheelite has been mined in small quantity in Idaho, 
Alaska, California, Nevada, Arizona, and New Mexico, as a source of 
tungsten, but most of this element has heretofore been produced from 
other compounds, mainly wolframite. In 191 3 a few hundred tons of 
scheelite concentrates were produced in the Atolia district, California, 
and the Old Hat district, near Tucson, Ariz. At present (1916) it is 
being produced in large quantity near Bishop, Inyo Co., Cal. 

Stolzite (PbW04) is completely isomorphous with wulfenite. Its 
crystals, which are pyramidal or short columnar, are mainly combina- 
tions of ooP(no), P(iii), 2P(22i) and oP(ooi). Their axial ratio is 
1 : 1.5606. 

The mineral is gray, brown, green or red. It is translucent and 
has a white streak. Its hardness is 2.75-3 an( ^ * ts S P- 6 r - 7-87-8.23. 
Its refractive indices for yellow light are: w = 2.2685, € = 2.182. 

Before the blowpipe it decrepitates and melts to a lustrous crystal- 
line globule. The bead with microcosmic salt in the reducing flame 



CHROMATES, TUNGSTATES AND MOLYBDATES 257 

is blue when cold; in the oxidizing flame it is colorless. The mineral 
is decomposed by HNO3 leaving a yellow residue of WO3. Crystals 
have been made by fusing sodium tungstate and lead chloride. 

Its principal localities are the tin-bearing veins at Zinnwald, Bo- 
hemia; the copper veins in Coquimbo, Chile; and Southampton, Mass., 
where it is associated with other lead compounds. 

Wulfenite (PbMoCU) 

Wulfenite is the only raolybdate of importance that occurs as a 
mineral. Its formula demands 39.3 M0O3 and 60.7 PbO. Calcium 
sometimes replaces a part of the Pb and tungsten a part of the Ma 

Wulfenite is hemihedral and hemimorphic (tetragonal pyramidal 
class). Its crystals are more frequently tabular than those of scheelite, 
and they are usually very thin. 

The mineral, however, occurs also in pyramidal and prismatic crys- 
tals which, in some cases, exhibit distinct hemimorphism. Their axial 




<^i 




Fig. 147. Fig. 148. 

Fig. 147. — Wulfenite Crystal with 00 p 00 , 100 (a) and ^P 00 , 1.0.12 (0). 
Fig. 148. — Wulfenite Crystal with oP, 001 (c); JP°o t 102 (w); P«, ioi (e); 

P, hi (n) and JP, 113 (5). 

ratio is a : c=i : 1.5777. The most common forms found on its crys- 
tals are: oP(ooi), P(in), |-^— ^(320), £P(ii3) and P 00(101) (Fig. 

147 and 148). The angle 1 1 1 A Ti 1 = 8o° 22'. 

The cleavage, parallel to P, is very smooth, and the fracture is con- 
choidal. The mineral is brittle. Its hardness is about 3 and specific 
gravity about 6.8. Its luster is resinous or adamantine, and its color 
orange-yellow, olive-green, gray, brown, bright red or colorless. Its 
streak is white and it is transparent. For red light, co= 2.402, €= 2.304. 

Before the blowpipe wulfenite decrepitates and fuses readily. With 
salt of phosphorus it gives the molybdenum beads. With soda on 
charcoal it yields a lead globule. When the powdered mineral is evap- 
orated with HC1 molybdic oxide is formed. On moistening this with 
water and adding metallic zinc an intense blue color is produced. 

Wulfenite is distinguished from lanadinite (p. 271), by crystalliza- 
tion, by the test for chlorine (vanadinite) and the test for tungsten. 



258 . DESCRIPTIVE MINERALOGY 

Synthesis. — Wulfenite crystals have been produced by melting 
together sodium molybdate and lead chloride. 

Occurrence and Localities. — The mineral occurs in the oxidized zone 
of veins of lead ores at some of the principal lead occurrences in Europe, 
end in the United States near Phoenixville, Pennsylvania; in the Organ 
Mountains, New Mexico; at the mines in Yuma County, Arizona; at 
the Mammoth Mine, in Pinal County in the same State, and at many 
other of the lead mines in the Rocky Mountain states. 

Uses. — Wulfenite is an important source of molybdenum, but, 
because of the few uses to which this metal is put, the amount of wulfen- 
ite mined annually is very small. 



~ * • 



WOLFRAMITE GROUP 

Wolframite ((Fe Mn)W0 4 ) 

Wolframite is the name given the isomorphous mixture of the man- 
ganese and iron tungstates that occur nearly pure in some varieties 
of the minerals hiibnerite and ferberite. 

The mixture of the iron and manganese molecules is more common 
than either alone, consequently wolframite is the commonest member of 
the group. The properties of all three minerals, however, are so nearly 
alike that they must be distinguished by chemical analysis. 

The name wolframite is usually applied to mixtures of the tungstates 
in which the proportion of Fe to Mn varies between 4 : 1 and 2 : 3, or 
between 9.5 per cent and 18.9 per cent of FeO and 14 per cent and 
4.7 per cent of Mn02. 

It has recently been suggested that the name ferberite be limited 
to mixtures containing not more than 20 per cent of the hiibnerite mole- 
cule and the name hiibnerite to those containing not more than 20 per 
cent of the ferberite molecule. This would leave the name wolframite 
for mixtures containing more than 20 per cent of both FeW04 and 

MnW0 4 . 

Analyses of specimens of hiibnerite (I), wolframite (II and III) 
and ferberite (IV) follow: 

WO3 FeO MnO CaO Other Total 

I. Ellsworth, Nye Co., Nev. . . 74.88 .56 23.87 .14 .16 99.61 

II. Sierra Cordoba, Argentine . . 74.86 13.45 11.02 ... 1.22 100.55 

III. Cabarrus Co., N. C 75-79 19.80 5.35 .32 tr 101.26 

IV. Kimbosan, Japan 75-47 24.33 •••■ tr tr 99.80 

All members of the group crystallize in the monoclinic system 
(prismatic class) with axial ratios as follows: 



CHROMATES, TUNGSTATES AND MOLYBDATES 259 



Ferberite a : 


:b: 


^=.8229 : 


1 : 


.8463. 


0- 


= 89° 


38' 


Wolframite 




= .8300 : 


1 : 


.8678. 


0= 


= 89° 


38' 


Hiibnerite 




= •8315^ 


1 : 


.8651. 


0= 


= 89° 


38' 




Fig. 149. — Wolframite Crys- 
tal with 00 P, no (m); 
00 P2, 210 (/); oop56, 
100 (a); -JP06, 102 (0; 
P«,on (/); — 2P2, 121 
(a); +JP« , 102 (>') and 
— P, in (w). 



The crystals are prismatic or cubic in habit and are bounded by 
ooP(no), 00 Poo (100), and two or more of the following: oP(ooi), 

00 P 06 (OIO), 00 P5(2I0), P ob (oil), £P 6b (T02), -£P 56 (102), -P(lll), 

— 2P2(i2i) and +2P00 (102) (Fig. 149). The 
angle 110A110 for ferberite=78° 51', for wol- 
framite 79 23', and for hiibnerite 79 29'. 
Twins are fairly common, with 00 P 60 (100) 
the twinning plane. Cleavage is perfect 
parallel to 00 P ob (010). The minerals also 
occur in lamellar and granular masses. 

Hiibnerite is* brownish red to black and 
translucent, wolframite is black and trans- 
lucent only on thin edges, and ferberite is 
black and opaque. The streak is yellow to 
yellowish brown in hiibnerite and brown or 
brownish black in ferberite, with the streak 
of wolframite between. 

Wolframite is brittle, has a hardness of 
5-5.5, a specific gravity of 7.2-7.5, and a submetallic luster. Before 
the blowpipe it fuses to a globule which is magnetic. Fused with 
soda and niter on platinum it gives the bluish green manganate. The 
salt of phosphorus bead is reddish yellow when hot and a paler tint 
when cold. In the reducing flame the bead becomes dark red. If 
the mineral is treated first on charcoal with tin its bead assumes a 
green color on cooling. The mineral dissolves in aqua regia with 
the production of the yellow tungsten trioxide. When treated with 
concentrated H2SO4 and zinc it yields the blue tungsten reaction. 

Crystals of wolframite are easily distinguished from crystallized 
columbite (p. 293), samarskite (p. 295), and uraninite (p. 297), by dif- 
ferences in crystallization. Massive wolframite is distinguished from 
massive forms of the other three minerals by its more perfect cleavage 
and by the reactions with the beads. Uraninite, moreover, contains 
lead. Wolframite is distinguished from black tourmaline (p. 434) by 
the differences in specific gravity. 

Occurrence and Origin. — Wolframite usually occurs in veins with tin 
ores, and in quartz veins with various sulphides, and in pegmatite. 
Its origin is probably pneumatolytic. 



260 DESCRIPTIVE MINERALOGY 

Localities. — Wolframite is found in all tin-producing districts, espe- 
cially at Zinnwald, Schneeberg and Freiberg, in Germany; at Ner- 
chinsk, in Siberia; in Cornwall, England; at Oruro, in Bolivia, and at 
various points in New South Wales, Australia. 

In the United States it occurs at Monroe, Conn.; near Mine La 
Motte, Missouri; near Lead, South Dakota, where it impregnates a 
sandy dolomite, and at Htil City in the same State in quartz veins, 
sometimes containing cassiterite; in Boulder Co., Colorado, in veins 
in granite (ferberite); near Butte, Montana, in quartz veins carry- 
ing silver ores (hubnerite) ; and the quartz-cassiterite veins near Nome 
and on Bonanza Creek, in Alaska; and in quartz veins at various 
points in Washington, Idaho, California, Nevada, New Mexico and 
Arizona. At some of these localities the mineral is more properly 
hubnerite. 

One or another of the three has been mined in Colorado, Nevada, 
South Dakota, Montana, Washington, California, Arizona, and New 
Mexico, but the total production has never been large. Some of the 
ore shipped has been obtained from placers along streams that drain 
regions containing the mineral in veins, but most of it has been obtained 
from vein rock which is crushed and concentrated. 

Uses. — These three minerals constitute the principal source of tung- 
sten used in the arts. The uses of the metal are referred to under 
scheelite. 

Production. — The total production of concentrates containing 60 
per cent WO3 in the United States during 1913 was 1,525 tons, valued 
at $640,500. Of this, 953 tons were ferberite from Boulder Co., 
Colorado. A little hubnerite was produced in the Arivica region, in 
southeast California, at Dragoon, Arizona, at Round Mountain, Nevada, 
and on Paterson Creek, Idaho. In addition, there were imported 
$86,000 worth of tungsten-bearing ores and $143,800 worth of tung- 
sten metal and ferro-tungsten. The world's production of tungsten ore 
in 1912 was 9,115 tons. 



CHAPTER XIV 

THE PHOSPHATES, ARSENATES AND VANADATES 

The phosphates are salts of phosphoric acid, H3PO4, the arsenates 
of the corresponding arsenic acid, H3ASO4, and the vanadates of the 
corresponding vanadic acid, H3VO4. The phosphates are by far the 
most important as minerals. They are easily distinguished by yielding 
phosphine, H3P, upon igniting with metallic magnesium and moistening 
the resulting MgaP 2 with H 2 or HC1 (Mg 3 P 2 +6HCl = 3MgCl 2 + 
2PH3). The gas is recognized by its disagreeable odor. The arsenates 
are detected by the test for arsenic. 

The arsenates, phosphates and vanadates form groups of isomor- 
phous compounds, the most important of which is the apatite group. 
Those occurring as minerals are divisible into several subgroups, of 
which the following six contain common minerals, viz.: (1) anhydrous 
(a) normal salts, (b) basic salts and (c) acid salts, and (2) hydrous 
(a) normal salts, (b) basic salts and (c) acid salts. 

A number of the phosphates and arsenates are of value commercially 
either because of the phosphorus they contain; because they are sources 
of valuable metallic salts; because they serve to indicate the presence 
of other valuable compounds; or because they possess an ornamental 
character. 

Nearly all the phosphates are transparent or translucent and all are 
nonconductors of electricity or are very poor conductors. 

ANHYDROUS PHOSPHATES, ARSENATES AND 

VANADATES 

NORMAL PHOSPHATES, ARSENATES AND VANADATES 

The minerals belonging in this class of compounds are not as numer- 
ous as the basic salts, but some of them are of great value. The class 
includes phosphates of yttrium, the alkalies, beryllium, cerium, mag- 
nesium, iron and manganese and a group of isomorphous phosphates, 
arsenates and vanadates — the apatite group — in which a haloid radicle 
replaces one of the hydrogen atoms of the acids. Apatite, the prin- 
cipal member of the group, is an important source of phosphoric acid. 

261 



262 DESCRIPTIVE MINERALOGY 

Triphylite— (Li(Mn Fe)P0 4 )— Lithiophilite 

Triphylite is the name usually applied to the isomorphous mixture 
of LiFeP04 and LiMnPCU, in which the manganese molecule is present 
in small quantity only. The mixture containing a large excess of the 
manganese molecule is called lithiophilite. 

The pure triphylite molecule contains FeO=45.5 P er cent > Li20 
= 9.5 per cent and P20s=45 per cent. The pure lithiophilite molecule 
consists of 45.1 per cent MnO, 9.6 per cent Li20 and 45.3 per cent 
P20 5 . 

Both substances are orthorhombic (bipyramidal class), with an axial 
ratio approximating .4348 : 1 : 5265. Crystals are rare and not well 
developed. They are usually rough prisms bounded by 00 P 06 (010), 
oP(ooi), ooP(no), ooP2(i2o) and 2P06 (021). The minerals usually 
occur massive, or in irregular, rounded crystals, with two very dis- 
tinct cleavages. 

Both minerals are transparent to translucent, both have a white 
streak, and both are vitreous to resinous in luster. Their hardness is 
about 4.5-5 and sp. gr. about 3.5. Triphylite is greenish gray to blue, 
and lithiophilite pink, yellow or brown. The refractive indices for 
light brown lithiophilite are: a= 1.676, £=1.679, 7=1.687; those for 
blue triphylite are a trifle higher. 

When heated in closed tubes both compounds are apt to turn dark. 
They fuse at a low temperature (1.5) and color the flame crimson. In 
the case of triphylite the crimson streak is bordered by the green of iron. 
Lithiophilite gives the reactions for Mn. Most specimens give reac- 
tions for all these metals — Fe, Mn and Li. Both minerals are soluble 
in HC1. 

The two minerals are distinguished from other compounds by their 
reactions for phosphorus and lithium, and from each other by the reac- 
tions for Fe and Mn. 

Occurrence. — They usually occur as primary constituents of coarse 
granite veins. They are associated with beryl, tourmaline and other 
pneumatolytic minerals and with secondary phosphates, which are 
presumably weathering products of the primary phosphates. 

Localities. — Both minerals occur at a number of points associated 
with other lithium compounds, especially spodumene (p. 378). In this 
country triphylite has been found at Peru, Maine; Grafton, New 
Hampshire, and Norwich, Massachusetts; lithiophilite at Branchville, 
Connecticut, and at Norway, Maine. 

Neither of the minerals possesses a commercial value at present. 



PHOSPHATES, ARSENATES AND VANADATES 263 



Beryllonite (NaBeP0 4 ) 

Beryllonite is a comparatively rare mineral occurring at only a few 
places and always in crystals or in crystalline grains. 

Its composition is 24.4 per cent Na20, 19.7 per cent BeO and 55.9 
per cent P2O5. 

Its crystals are orthorhombic (bipyramidal class), with an axial 
ratio .5724 : 1 : .5490. They are short pyramidal or tabular in habit, 
often exhibiting a pseudohexagonal symmetry. Most crystals are 
highly modified with oP(ooi), ooPw(ioo), 00 P 66 (010), P 66(101) 
and 2P2(i2i), the principal forms. Twins are common, with 00 P(no) 
the twinning plane. The crystal faces are frequently strongly etched. 

The mineral is white to pale yellow. It has a vitreous luster, 
except on oP(ooi), where the luster is sometimes pearly. It possesses 
four cleavages, of which the most perfect is parallel to oP(ooi). That 
parallel to ooPw (100) is distinct, but the others are indistinct. Its 
hardness is 5.5-6 and its density 2.845. ^ s fracture is conchoidal. 
Crystals often contain numerous inclusions of water and liquid CO2 
} arranged in lines parallel to c. Its refractive indices for yellow light 
are: 0=1.5520,13=1.5579,7=1.5608. 

Beryllonite decrepitates and fuses in the blowpipe flame to a cloudy 
glass, at the same time imparting to the flame a yellow color. It is 
slowly soluble in HC1, and gives the phosphorus reaction with mag- 
nesium. 

It is distinguished from most other colorless transparent minerals 
by the reaction for phosphorus; from other colorless phosphates by its 
crystallization and the sodium flame test. 

Occurrence and Localities. — The best known occurrence of beryllo- 
nite in the United States is Stoneham, Maine, where it is found in the 
debris of a pegmatite dike associated with apatite (p. 266), beryl (p. 359), 
and other common constituents of pegmatites. It originally existed 
implanted on the walls of cavities in the pegmatite and was apparently 
the result of pneumatolytic processes. 

Use. — The mineral is used to some extent as a gem stone. 

Monazite ((Ce Di La)P0 4 ) 

Monazite is the principal source of certain rare earths that are used 
in manufacturing gas mantles. Although it occurs as small grains and 
crystals in certain granites it is found in commercial quantities only in 
the sands of streams. 



264 DESCRIPTIVE MINERALOGY 

The mineral is a phosphate of the metals cerium, lanthanum, praseo- 
didymium and neodidymium in most cases combined with the silicate of 
thorium. Its composition may be represented by the formula 

*((Ce • La • Di)P0 4 )+y(ThSi0 4 ), 

in which the proportion of the second constituent varies from a trace to 
an amount yielding 20 per cent ThCfe. Since this is not constant in 
quantity it is not to be regarded as an essential portion of the com- 
pound. It is probable that in monazite we have to do with a solid 
solution of cerium and thorium phosphates, thorium silicate and oxides 
of the rare metals. 

Monazite is monoclinic with a : b : £=.9693 : 1 : .9255 and j5= 
76 20'. Crystals are usually prismatic with the pinacoids 00 P * (100), 
00 Poo (010), the prism ooP(no), the two domes — P 60(101) and 
+P66(ioT) and the pyramids — P(ni) and +P(nT). They are 

often flattened parallel to the orthopinacoid 
(Fig. 150). The angle iioAiTo=86° 34'. 

Their cleavage is perfect parallel to oP. 
The color of the mineral is gray, yellow, red- 
dish, brown or green. It is usually transpar- 
ent or translucent and sometimes opaque. It 
is brittle, has a white streak, and a resinous 
luster. Its hardness is 5-5.5 and its sp. gr. 
Fig. 150.— Monazite Crys- 4.7-5.3, varying with the proportion of thorium 
tal with oop do, 100(a); present> j^ refractive indices for yellow 

00P, no (*w); 00 P2, ... 00 

' „w ,.v light are: a= 1.7038, 7=1.8452. 

120 («); 00 P 00 , 010 (6); ° m § /° ' ' ^ D 

-Poo, 101 («/)• +P00, The mineral is infusible. Before the blow- 
10T (x) and P, nT (v). pipe it turns gray, and when moistened with 

H2SO4 it colors the flame bluish green. It is 
difficultly soluble in HC1 and HNO3. Most specimens are strongly 
radioactive. 

Synthesis. — Crystals of monazite have not been prepared, but crys- 
tals of cerium phosphate similar to those of monazite have been made 
by heating to redness a mixture of cerium phosphate and cerium chloride. 
Occurrence and Origin. — Monazite occurs as the constituent of cer- 
tain granites and granitic schists in small crystals scattered among the 
other components. In this form it is a separation from the granitic 
magma. When the granites are broken down to sand by weathering 
the monazite is freed and because of its specific gravity it concentrates 
in stream channels. 

Localities. — Although the mineral is fairly widespread in the rocks, 




PHOSPHATES, ARSENATES AND VANADATES 265 



it is concentrated into commercial deposits at only a few places. The 
most important of these are in southeastern Brazil, in Norway, and in a 
belt 20 to 30 miles wide and 150 miles long extending along the east side 
of the Appalachian Mountains from North Carolina into South Carolina. 

The mineral has also been reported from many points in ten coun- 
ties in Idaho. Near Centerville it may be in sufficient quantity to be 
of commercial importance. 

Preparation. — Monazite is separated from the valueless sand in 
which it is found, by washing, and the residues thus resulting are further 
concentrated by a magnetic process. The commercial concentrates 
produced in this way usually contain from 3 to 9 per cent TI1O2, and 
their price varies accordingly. 

. Production and Uses. — Monazite is the chief source of thorium oxide 
used in the manufacture of incandescent gas mantles. Formerly it was 
produced in large quantity in the Carolinas, the production in 1909 
amounting to 542,000 lb., valued at $65,032, and in 1905 to 
1,352,418 lb., valued at $163,908. All of this was manufactured into 
the nitrate of thorium in this country and the amount made was 
not sufficient to meet the domestic demand. Consequently, large quan- 
tities of the nitrate were imported. In 1910-11 mining of the mineral 
in the Carolinas ceased and all the monazite needed has been imported 
since then. The imports of thorium nitrate for 191 2 were 117,485 lb., 
valued at $225,386 and of monazite, an amount valued at $47,334. 



a^ 



Xenotime (YP0 4 ) 

Xenotime, though essentially an yttrium phosphate, usually contains 
erbium and in some cases cerium. 
It occurs in tetragonal crystals 
and in rolled grains. Its axial 
ratio is 1 : .6177 and the angle 
111A1T1 =55° 30'. Its crystals 
are octahedral or prismatic and 
are bounded by 00 P(i 10), P(i 11), 
and in some cases by 00 P 00 (100) 
and 2P 00 (201) (Fig. 151). Their 
cleavage is perfect parallel to Fig. 151.— Xenotime Crystals with °op, i XO 

■n/ \ »m. • 1 • u ( OT )> P- I" W, and 00 Poo; ioo (a). 

ooP(no). The mineral is brown, w ' ' w 

pink, gray or yellow. Its streak is a pale shade of the same color. 
It is opaque and brittle. Its luster is vitreous or resinous; its hardness 
4-5 and specific gravity 4.5. Its indices of refraction are: e=i.8i, 

tt=1.72. 





266 DESCRIPTIVE MINERALOGY 

Xenotime is infusible, insoluble in acids and with difficulty soluble 
in molten microcosmic salt. It is distinguished from zircon by its 
cleavage and inferior hardness. 

A variety of xenotime containing a small percentage of sulphates is 
known as hussakite. 

The mineral occurs in pegmatite veins, in granites and in the sands 
of streams. It is found in pegmatite veins at Hittero, Moss, and other 
places in Norway; at Ytterby, Sweden; in the granites of Minas Geraes, 
Brazil, and in the gold washings at Clarksville, Georgia, and many places 
in North Carolina, and in pegmatite veins in Alexander County in the 
same State. 

APATITE GROUP 

The apatite group consists of a number of phosphates, arsenates and 
vanadates in which fluorine or chlorine takes the place of the hydroxyl 
in basic compounds. Thus, jluorapatite is Ca4(CaF)(P04)3 and chlor- 
apatite Ca4(CaCl)(P04)3- The group contains a number of important 
minerals, of which apatite is by far the most valuable. These minerals 
are isomorphous, all crystallizing in the hemihedral division of the hex- 
agonal system (hexagonal bipyramidal class). The names, composi- 
tions and axial ratios of the most important are as follows: 

Fluor apatite Ca4(CaF)(P04)3 a: c=i : .7346 

Chlorapatite Ca4(CaCl)(P0 4 )3 a : c=i : .7346+ 

Pyromorphite Pbi(PbCl)(P04)3 a : c=i : .7293 

Mimetite Pbi(PbCl)(As04)3 a : c=i : .7315 

Vanadinite Pb 4 (PbCl)(V0 4 )3 a : c=i : .7122 

Apatite (Ca4(Ca(F-Cl))(P0 4 ) 3 ) 

Although fluorapatite and chlorapatite are distinct compounds with 
slightly different properties, nevertheless, because of the difficulty of 
discriminating between them without analyses, the name apatite is 
commonly applied to both. This is justified because of the fact that the 
two compounds are completely isomorphous, and the mineral as it 
usually occurs is a mixture of both. The ideal molecules comprising 
the two varieties of apatite have the following compositions: 

Fluorapatite CaO=55-5, F=3.8, P20s = 42.3 
Chlorapatite CaO=53-8, Cl = 6.8, P20s = 4i.o 

Apatite is found in well defined crystals, sometimes very large. 
These have a holohedral habit, but etch figures on their basal planes 



PHOSPHATES, ARSENATES AND VANADATES 267 

reveal the grade of symmetry of pyramidal hemihedrism. The min- 
eral occurs also massive, in granular and fibrous aggregates and less 
commonly in globular forms and as crusts. 

The crystals are usually columnar or tabular, with the hexagonal 
prism or pyramid well developed. Although in some cases highly 
modified, most crystals contain only the oo P(iolo), P(ioTi) and oP(oooi) 
planes prominent, though £P(iol2) and 2P2(ii2i) are not uncommon as 
small faces (Figs. 152 and 153). Their cleavage is indistinct, and their 
fracture often conchoidal. 

Apatite may possess almost any color. In a few cases the mineral is 
colorless or amethystine and transparent, but in most cases it is trans- 
lucent or opaque and white, green, bluish, brown or red. Its streak is 






Fig. 152. 



Fig. 153. 



Fig. 152. — Apatite Crystals with 00 P, 1010 (w); P, 1011 (.r); oP, oooi (c); JP, 

1012 (r) and 00 P2, 1120 (a). 

Fig. 153. — Apatite Crystal with m, x, r and c as in Fig. 152 and 2P, 2021 (y); 4PJ, 
1341 00; 3P}> i 2 3i GO; 2P2, 1121 (5); P2, 1122 (») and 00 p|, 1230 (A). 



white and its luster vitreous to resinous. Its hardness is 4.5-5 an ^ sp. 
gr. between 3.09 and 3.39. The refractive indices of fluorapatite for 
yellow light are: «= 1.6335, €=1.6316 and of chlorapatite, o>= 1.6667. 
Many specimens are distinctly phosphorescent. Nearly all fluoresce in 
yellowish green tints, and all are thermo-electric. 

Apatite fuses with difficulty, tinging the flame reddish yellow. The 
chlorapatite melts at 1530 and the fluorine variety at 1650 . When 
moistened with H2SO4 all varieties color the flame pale bluish green, 
due to the phosphoric acid. Specimens containing chlorine give the 
brilliant blue color to the flame when fused in a bead of microcosmic 
salt that has been saturated with copper oxide. Specimens containing 
fluorine etch glass when fused with this salt in an open glass tube. 
The mineral also yields phosphine when ignited with magnesium, and 
it dissolves in HC1 and HNO3. 



268 DESCRIPTIVE MINERALOGY 

Apatite is much softer than beryl (p. 359), which it closely resembles 
in appearance. It is distinguished from calciU by lack of effervescence 
with acids and from other compounds by the phosphorus reaction. 

The varieties of the mineral recognized by distinct names are: 

Ordinary apatite, crystals or granular masses. 

Manganapatite, in which manganese partly replaces the Ca of ordi- 
nary apatite. This is dark bluish green. 

Fibrous, concretionary apatite. Known also as phosphorite. 

Osteolite. The earthy variety. 

Phosphate rock. A mixture of apatite, phosphorite, several hydrous 
carbonates and phosphates of calcium, and fragments of bone and 
teeth. It is more properly a rock with a brecciated and concretionary 
structure. The composition of typical deposits is represented by the 
following analysis of hard rock phosphate from South Carolina: 

CaO P2O5 C0 2 Fe 2 03 AI2O3 MgO Insol. Undet. H 2 Moist. 
50.08 38.84 .65 .96 3.07 .30 .49 2.46 2.96 .07 

Guano is a mixture of various phosphates, both hydrous and an- 
hydrous, calcite and a number of other compounds. It is rather a rock 
than a mineral, as it has no definite composition. 

Syntheses. — Crystals of fluorapatite have been made by fusing 
sodium phosphate with CaF 2 and by heating calcium phosphate with a 
mixture of KF and KC1. 

Origin. — The crystallized apatite was formed by direct separation 
from igneous rock magmas and by pneumatolytic action upon limestone. 
The phosphorite variety and the phosphate in phosphate rock were 
probably produced by the solution of calcium phosphate and its later 
deposition from solution — the original phosphate having been furnished 
in many cases by the shells of mollusca, and by the action of phosphoric 
acid produced by the decay of organisms upon limestone. In many 
cases phosphorite accumulated as a residual deposit in consequence of 
the solution of the calcite and dolomite from phosphatic limestone, 
leaving the less soluble phosphate as a mantle on the surface. 

Occurrence. — The mineral occurs in microscopic crystals- as a com- 
ponent of many rocks, as large crystals in metamorphosed limestones, 
as a component of many coarse-grained veins, especially those composed 
of coarse granite and those in which cassiterite, magnetite, tourmaline, 
and other pneumatolytic minerals are found. At a number of places 
aggregates of apatite and magnetite or ilmenite occur in such large 
masses as to be worthy of being called rocks. An impure apatite in 
concretionary and fibrous forms also occurs in thin beds covering large 



PHOSPHATES, ARSENATES AND VANADATES 269 

areas. It is often mixed with other phosphates, with the bones and 
teeth of animals and with other impurities. This is the well known 
phosphate rock or phosphorite. 

Localities. — Crystallized apatite is so widely spread that it is useless 
to mention its occurrences. It is mined at Kragero and near Bamle, 
in Norway; at various points in Ottawa County in Quebec, and in 
Frontenac, Lanark and Leeds Counties in Ontario; and at Mineville, 
New York. Rock phosphate is found in extensive beds on the west 
side of the peninsula of Florida, in South Carolina, North Carolina, 
Alabama, Tennessee, Wyoming, Idaho, Utah and Arkansas. A mixture 
of apatite and ilmenite {nelsonite) , occurs as dikes in Nelson and 
Roanoke Counties, Virginia. 

Uses. — The principal use of apatite and phosphate rock is in the 
manufacture of fertilizers. The rock (or crushed apatite) is treated 
with H2SO4 to make an acid phosphate which is soluble in water. Am- 
monia or potash, or both, are added to the mass and the compound is 
sold as a superphosphate. The purest varieties are treated with H2SO4 
in sufficient quantity to entirely decompose them, CaS04 and H3PO4 
being formed. The latter is drawn off and mixed with additional high- 
grade rock and the mixture is known as concentrated phosphate. Super- 
phosphates are manufactured in large quantities in the United States 
and the concentrated phosphates in Europe. Unfortunately, for the 
latter use the best grades of apatite or rock phosphate are required, and 
consequently the best grades of rock produced in the United States are 
exported and thus lost to American farmers. 

Production. — The world's production of apatite and phosphate rock 
during 191 2 was as follows: 

United States 3,020,905 tons, valued at $11,675,774 

Tunis 2,050,200 tons, valued at 7,500,000 

Christmas Island. . 159,459 tons, valued at 2,024,036 

France ... 313,151 tons, valued at 1,169,400 

Algeria 207,111 tons, valued at 759,455 

Belgium 203,110 tons, valued at 316,703 

Other countries 65,000 tons, valued at 280,000 

For the United States production of 191 2 the statistics are: 

Florida 2,407,000 tons, valued at $9,461,000 

Tennessee 423,300 tons, valued at 1,640,500 

South Carolina.. . . 131,500 tons, valued at 524,700 

Other States 11,600 tons, valued at 49,200 



270 DESCRIPTIVE MINERALOGY 

The total production was 3,020,905 tons, valued at $11,675,774.00, 
of which 1,206,520 tons, valued at $8,996,456.00 were exported. Par- 
tially offsetting this, there were imported guano, apatite and other phos- 
phates to the value of about $2,000 000. 

Pyromorphite (Pb 4 (PbCl)(P0 4 )3) 

In composition pyromorphite is PbO, 82.2 per cent, P2O5, 15.7 per 
cent and CI, 2.6 per cent, but there are usually present also CaO and 
AS2O5. 

The mineral is completely isomorphous with apatite. Its crystals 
are smaller and simpler than those of apatite, but they have the same 
habit. Their axial ratio is a : c=i : .7293. This increases to 1 : .7354 
in varieties containing calcium. 

Crystals are often rounded into barrel-shaped forms, and frequently 
are mere skeletons. Tapering groups of slender crystals in parallel 
growths are also common. Their cleavage is parallel to the 00 P(no) 
faces, and their fracture is feebly conchoidal. The mineral also occurs 
in globular, granular and fibrous masses. 

Pyromorphite is translucent. It is brittle, has a hardness of 3.5-4 
and a density of about 7. Its luster is resinous and color usually green, 
yellow, brown or orange. Some varieties are gray or milk-white. Its 
streak is white. Its refractive indices for yellow light are: «= 2.0614, 
€= 2.0494. The mineral is distinctly thermo-electric. 

When heated in the closed tube pyromorphite gives a white subli- 
mate of lead chloride. It fuses easily, coloring the flame bluish green. 
When heated on charcoal it melts to a globule, which crystallizes on 
cooling and yields a coating which is yellow (PbO) near the assay and 
white (PbCfe), at a greater distance from it. When fused with Na2C03 
on charcoal a globule of lead results. The mineral also gives the CI and 
P reactions. The mineral is soluble in HNO3. 

Pyromorphite is recognized by its form, high specific gravity and its 
action when heated on charcoal. 

Synthesis. — Crystals have been obtained by fusing sodium phosphate 
with PbCl 2 . 

Occurrence. — The mineral occurs principally in veins with other lead 
ores, especially in the zone of weathering. It also exists in pseudomorphs 
after galena. 

Localities. — It is found in all lead-producing regions, especially in 
the upper portions of veins. It occurs in particularly good specimens 
at Pribram, Bohemia; at Ems, in Nassau; in Cornwall, Devon, Derby- 



PHOSPHATES, ARSENATES AND VANADATES 271 

shire and Cumberland, England; at Phoenix ville, Pennsylvania, and 
at various other points in the Appalachian region. 

Uses. — Pyromorphite alone possesses no commercial value, but it 
is mined with other compounds of lead as an ore of this metal. 

Mimetite (Pb 4 (PbCl)(As0 4 )3) 

Mimetite, or mimetesite, resembles pyromorphite in its crystals and 
general appearance, and many of its properties. Its color, however, is 
lighter and its density slightly greater. It occurs in crystals, in fila- 
ments, and in concretionary masses and crusts. Its axial ratio is 
i : .7315 and its refractive indices for yellow light are: £0=2.1443, e 
= 2.1286. 

The formula for mimetite demands 74.9 per cent PbO, 23.2 per cent 
AS2O5 and 2.4 CI. Usually a portion of the lead is replaced by CaO and 
a portion of the As by P. 

Mimetite fuses more easily than pyromorphite. It differs from this 
mineral in yielding arsenical fumes when heated on charcoal. More- 
over, when heated in a closed tube with a fragment of charcoal it coats 
the walls of the tube with metallic arsenic. 

Occurreftce and Localities. — It occurs with other lead minerals in 
veins, usually coating them either as crusts or as a series of small crys- 
tals. It is found at Phoenix ville, Pennsylvania; in Cornwall, England; 
at Johanngeorgenstadt, in Germany; at Nerchinsk, Siberia; at Lang- 
ban, in Sweden, and at a number of other places. It is, however, not 
as common as the corresponding phosphorus compound. 

Uses. — It is mined with other compounds as an ore of lead. 

Vanadinite (Pb 4 (PbCl)(V0 4 )) 3 

Vanadinite is the most widely distributed of all the vanadium min- 
erals. It usually occurs in small bright red prismatic crystals implanted 
on other minerals, or on the walls of crevices in rocks. It is one of the 
sources of vanadium. 

Its theoretical composition is as follows: PbO =78.7 per cent, 
Vo05=i94 per cent and CI =2.5 per cent, but phosphorus and arsenic 
are often also present. When arsenic and vanadium are present in 
nearly equal quantities the mineral is known as endlichitc. 

Its crystals are hexagonal prisms and pyramids bounded by 
ooP(ioTo), oP(oooi), ooP2(ii2o), P(ioTi) and other forms, with an 
axial ratio 1 : .7122 (Fig. 154). Often the crystals have hollow faces 



272 DESCRIPTIVE MINERALOGY 

(Fig, 155)- Frequently they are grouped into pyramids like those of 
pyromorphite. The mineral occurs also in globules and crusts. 

Vanadinite is brittle, has a hardness of about 3 and a specific gravity 
of about 7. Its fracture is conchoidal. Its luster is adamantine or 
resinous and its color ruby red, brownish yellow or reddish brown. 
Its streak is white or light yellow. The mineral is translucent 
or opaque. Its refractive indices for yellow light are: (0=2.354, 
4=2.299. 

In the closed tube vanadinite decrepitates. It fuses easily on char- 
coal to a black lustrous mass which is reduced on being further heated 
in the reducing flame to a globule of lead. A white sublimate of PbCfe 
also coats the charcoal. The mineral, moreover, gives the flame test 



f^\ 



\^sJJ 



—Vanadinite Crystal with °°P, 1010 (in); oP, 1 



m- 



Fig. 155. — Skeleton Crystal of Vanadinite. 

for chlorine with copper. After complete oxidation of the lead by heat- 
ing in the oxidizing flame on charcoal the residue gives an emerald-green 
bead in the reducing flame with microcosmic salt and this turns to a 
light yellow in the oxidizing flame. The mineral is soluble in hydro- 
chloric acid. If to the solution a little hydrogen peroxide is added it 
will turn brown. The addition of metallic tin to this will cause it to 
turn blue, green and lavender in succession, in consequence of the reduc- 
tion of the vanadium compounds. 

Vanadinite is easily distinguished from most other minerals by its 
color. It is distinguished from other compounds of the same color by 
its crystallization and by the reactions for vanadium. 

Occurrence. — Vanadinite occurs principally in regions of volcanic 
rocks. It is probably a result of pneumatolytic processes. 

Localities. — Crystals are found at Zimapan, Mexico; Wanlockhead, 



PHOSPHATES, ARSENATES AND VANADATES 273 

England; Undenas, Sweden; in the Sierra de C6rdoba, Argentine, and in 
the mining districts of Arizona and New Mexico. 

Uses. — Vanadinite is an important source of vanadium, which is 
employed in the manufacture of certain grades of steel and bronze. 
Its compounds are, moreover, used as pigments and mordants. Most 
of the vanadium compounds produced in this country are obtained from 
other vanadium minerals, among them patronite — a mixture, of which 
the principal component is a sulphide (VS4) — and carnotite (p. 290), 
but vanadinite has been used abroad and also to a small extent in the 
United States. 

WAGNERITE GROUP 

This group, in chemical composition, is analogous to the apatite 
group. It includes a number of phosphates and arsenates containing a 
fluoride radical. The group is monoclinic (prismatic class), with an 
axial ratio which is approximately 1.9 : 1 : 1.5, with £=71° 50'. None 
of its members are important. The two most common ones are wag- 
nerite (Mg(MgF)P0 4 ), and triplite (Fe-Mn) ((Fe-Mn)F)P0 4 . 

Wagnerite occurs in massive forms and in large rough crystals, with 
imperfect cleavages parallel to 00 P <x> (100) and 00 P(no). Its crystals 
have an axial ratio of 1.9145 : 1 : 1.5059 with /3=7i° 53'. They are 
often very complex. The mineral is brittle. Its fracture is uneven. 
Its hardness is 5.5 and density 3.09. Its color is yellow, gray, pink or 
green. It is vitreous, translucent and has a white streak. Its refractive 
indices are: a= 1.569, 0= 1.570, 7 = 1.582. It fuses to a greenish gray 
glass and gives the usual reactions for fluorine and phosphoric acid. It 
is soluble in HC1 and HNO3, and heated with H2SO4 it yields hydro- 
fluoric acid. It occurs in good crystals near Werfen, Austria, and in 
coarse crystals near Bamle, Norway. 

Triplite is an isomorphous mixture of Fe(FeF)PC>4 and Mn(MnF)PC>4. 
It usually occurs massive, but is found in a few places in rough crystals. 
The mineral is dark brown or nearly black, is translucent to opaque, 
and has a yellowish gray or brown streak. It possesses two unequal 
cleavages perpendicular to one another and a weakly conchoidal frac- 
ture. Its hardness is 4-5.5 and specific gravity about 3.9. Its luster is 
resinous. Its intermediate refractive index is 1.660. 

Before the blowpipe triplite fuses easily (1.5) to a black magnetic 
globule. It reacts for Mn, Fe, F, and P2O5. It is soluble in HC1 and 
evolves hydrofluoric acid with H2SO4. It is found in coarse granite 



274 DESCRIPTIVE MINERALOGY 

veins at Limoges, France; Helsingfors, Finland; Stoneham, Maine; 
and Branchville, Connecticut. In all of its occurrences it appears to 
be pneumatolytic. 

BASIC PHOSPHATES AND ARSENATES 

The basic phosphates are those in which there is more metal present 
than sufficient to replace the three hydrogen atoms in the normal acid, 
H3PO4. This is due to the replacement of one or more of the hydrogen 
atoms by a group of atoms consisting of a metal and hydroxyl (OH). 
All yield water when heated in the closed tube. 

The principal basic phosphates are amblygonite, a source of lithium 
compounds, dufrenite and lazulite, neither of which is of economic im- 
portance, and libethenite^ a copper compound which occurs in compara- 
tively small quantities with other copper ores, and is mined with 
them. 

Olivenite is a basic copper arsenate corresponding to the phosphate 
libethenite. 

Amblygonite (Li(Al(F OH))P0 4 ) 

Amblygonite is an isomorphous mixture of the two compounds 
(AlF)LiP04 and (AlOHQLiPQi. It is an important source of lithium. 

The composition of the fluorine molecule is Al20a = 34.4 per cent, 
Li02=io.i per cent and P20s = 47.9 per cent, making a total of 105.3 
per cent from which deducting 5.3 per cent (0= 2F), leaves 100. Nearly 
always a portion of the F is replaced by OH and a part of the Li 
by Na. The pure Na(A10H)PC>4 is known Sisfremonttie, and the pure 
Li(AlOH)P04 as montcbrasite. 

The analysis of a specimen from Paia, California, gave: 



P«Os 


AltOa 


FeaOi 


MnO 


MgO 


Li,0 


NajO 


H.O 


0-P Total 


48.83 


33.70 


.12 


.09 


.31 


9.88 


.14 


595 


2.29 =»ioi.3i — .96 — 100.45 



The mineral forms large, ill-defined triclinic crystals (Fig. 156), and 
compact masses with a columnar cleavage. Crystals are very rare, and 
are poorly developed. Their axial ratio is .7334 : 1 : .7633. The 
cleavage pieces often show polysynthetic twinning lamellae parallel to 
'P' 00 (101) and /P y 00 (Toi). 

The cleavage of the mineral is perfect parallel to oP(ooi). Its 
fracture is uneven. It is brittle, has a hardness of 6 and a density of 
3.03. Its color is white, gray, or a very light tint of blue, pink or 
yellow. Its luster is vitreous, except on oP where it is pearly. Its 



PHOSPHATES, ARSENATES AND VANADATES 275 




ioo (a); oP, ooi (c); 
oo 'P, ilo (M);oop], 
no (m); oo^, 1 20 
(z) ; /P/ 00 , Toi (A) 
and a'P 00 , 021 (e). 



streak is white and it is translucent. Its refractive indices for yellow 
light are: a= 1.579, £=1.593, 7=1.597. 

In the closed tube at high temperature it yields water which reacts 
acid and corrodes glass. It fuses easily to an 
opaque white enamel. It colors the flame red 
with a slight fringe of green. When moistened 
with H2SO4 it tinges the flame bluish green. 
When finely powdered it dissolves readily in 
II2SO4 and with dfficulty in HC1. 

Amblygonite resembles in appearance many 
other minerals, especially spodumene (p. 378), 
and some forms of barite, feldspar, dolomite, etc. p IG IS 6— Amblygonite 
From spodumene it is distinguished by the phos- Crystal with 00 p 56 , 
phorus reaction and the acid water; from the 
others by its easy fusibility. 

Occurrence. — Amblygonite is found in granite 
and in pegmatite veins associated with other 
lithium compounds, tourmaline, cassiterite and 
other minerals of pneumatolytic origin. In all cases it also is probably 
a result of pneumatolytic action associated with the last phases of granite 
intrusions. 

Localities. — The mineral occurs near Penig, in Saxony; at Arendal, 
in Norway; at Montebras, France; at Hebron, Paris and Peru, Maine; 
at Branchville, Conn.; at Pala, in California; and near Keystone, in 
the Black Hills, South Dakota. 

Uses and Production. — The mineral is the principal source of lithium 
compounds in the United States. It is used in the manufacture of 
LiC03, which is employed as a medicine, in making mineral waters, in 
photography and in pyrotechnics. 

It has been mined in South Dakota and in California to the extent 
of a couple of thousand tons, valued perhaps at $20,000. 

Dufrenite (Feo(OH)3P0 4 ) 

Dufrenite, or kraurite, is a basic iron phosphate containing 62 per 
cent Fe2C>3, 27.5 per cent P2O5 and 10.5 per cent water. It may be 
regarded as a normal phosphate in which one H atom of H3PO4 has been 
replaced by the Fe(0H)2 group and two by the group Fe(OH), thus 

HO-Fe=P0 4 -Fe<^' 

It forms small orthorhombic crystals with a cubic habit that are rare. 
Their axial ratio is .3734 : 1 : .4262. It usually occurs massive, in 



276 



DESCRIPTIVE MINERALOGY 



nodules, or in fibrous radiating aggregates. The same substance is 
believed to occur also in the colloidal condition under the name delvauxite. 

The color of dufrenite varies from leek-green to dark green, which 
alters on exposure to yellow and brown. It is translucent to opaque, 
has a light green streak and is strongly pleochroic. Its hardness is 
3.5-4 and specific gravity about 3.3. 

In the closed tube it yields water and whitens. It fuses easily, color- 
ing the flame bluish green and yielding a magnetic globule. It is sol- 
uble in HC1 and in dilute H2SOt. 

It is recognized by its color and the presence in it of water, phos- 
phorus and iron. 

Localities and Origin, — The mineral has been observed at several 
points in Europe; at Allentown, New Jersey, and in Rockbridge County, 
Virginia. It is thought to be ptoduced by the weathering of other fer- 
ruginous phosphates. 

Lazulite ((Mg-Fe)(A10H) 2 (P0 4 )2) 

Lazulite is essentially an isomorphous mixture of the two com- 
pounds Mg(A10H) 2 (P0 4 )2 and Fe(AiOH) 2 (PO.i) 2 . There is also fre- 
quently present in it a little calcium. 
When the proportion of the two 
molecules present is as 2 : 1 the com- 
position becomes FeO=7.7; MgO 
= 8.5; Al 2 03 = 32.6; P 2 5 =454 an d 
H 2 0=5.8. 

The mineral occurs in blue pyram- 
idal crystals that are monoclinic 
(prismatic class), with the axial ratio 
= .9750 : 1 : 1.6483 and £=89° 14'. 
A u The predominant forms are +P(nT), 

Fig. 157.— Lazulite Crystals. A with — P(ni) and — P 00 (ioi)(Fig. 157^). 
-P, in (/>); +P, hi Wand P«, The angle in AITI = 79 40 , . Twins 
101 (/). B is the same combination are not common> Those most fre . 

twinned about 00 P 06 (100). with oP . . , A . . . 

, x # . , quently found are twinned about c 

(001) the composition face. M J 

as the twinning axis (Fig. 157B). 
It is found also massive and in granular aggregates. 

The cleavage of lazulite is not distinct. Its fracture is uneven. It 
is brittle, has a vitreous luster, is translucent or opaque, has an azure 
color and a white streak. Its hardness is 5 or 6 and its specific gravity 
about 3.1. Translucent crystals are strongly pleochroic in deep blue 
and greenish blue tints — the former when viewed along the vertical 




B 



PHOSPHATES, ARSENATES AND VANADATES 277 

axis. Their indices of refraction for yellow light are: a= i .603, 0= 1.632, 
7=1.639. 

In the closed tube lazulite swells, whitens and yields water. When 
heated in the blowpipe flame it whitens, falls to pieces and colors the 
flame bluish green. The white powder moistened with Co(NQs)2 and 
reheated regains its blue color. When moistened with H2SO4 and 
heated in the blowpipe flame it imparts to it a green blue color. It is 
infusible and is unacted upon by acids. 

Lazulite, when massive, closely resembles in appearance massive 
forms of some varieties of sodalite, haiiynite and lazurite (p. 333). The 
latter, however, are soluble in HC1. Moreover, none of them contains 
phosphorus. 

Occurrence. — The mineral occurs in quartz veins in sandstones and 
slates and is usually a product of metamorphism. It is sometimes, how- 
ever, found in serpentine rocks, with corundum, in which case it may be 
original. 

Localities. — Good crystals occur at Krieglach, in Styria; at Horrs- 
joberg, in Sweden, and in the United States at Crowder's Mountain, 
North Carolina, and on Graves Mountain in Georgia. 



OLIVENITE GROUP 

The olivenite group includes a number of basic copper, lead and 
zinc compounds of the general formula R"2(OH)R / "04 in which R" 
= Cu, Zn, Pb and R"'=As, P, V. The group is 
orthorhombic (bipyramidal class), with axial ratios 
approximating .95 : 1 : .70. The most important 
members of the group are the two copper min- 
erals, olivenite, Cu(CuOH) As04 and libethenite, 
Cu(CuOH)P0 4 . 

Olivenite occurs in fibrous, globular, lamellar, 
granular and earthy masses and in prismatic and 
acicular crystals bounded by 00 P(i 10), 00 P 6b (100), FlG - J 5 8 - — Olivenite 
ooP66(oio),P56(oii) andPob(ioi) (Fig. 158). CryStal mth °° P °° ' 
Their axial ratio is .9396 : 1 : .6726 and the angle 
1 10 A 1 To= 86° 26'. Their cleavage is poor. 

The mineral is some shade of green, brown, 
yellow or grayish white and its streak is olive-green 
in greenish varieties. It is transparent to opaque, is brittle, has a 
hardness =3, and a specific gravity =4.3. Its refractive indices for 




100 (a); 00 P, no 
(w); 00 Poo ,010(6); 
P 00 , 01 1 (e ) and 
P »o , 101 (»). 



278 DESCRIPTIVE MINERALOGY 

yellow light are about 1.83. Its luster is usually vitreous. Fibrous 
varieties are sometimes known as wood-copper. 

Olivenite fuses easily (2) to a mass that appears crystalline on cooling. 
It gives the usual reactions for H2O, Cu, and As. It is soluble in acids 
and in ammonia. 

It is associated with other copper compounds in some copper ores. 
Its origin is secondary in all cases. It occurs in the Tintic district, 
Utah, and in many copper veins in Europe and in South America. 

Libethenite occurs in compact or globular masses and in small 
crystals that resemble those of olivenite. Their axial ratio is .9605 : 
1 : .7019 and no A 110=87° 40'. 

The mineral is brittle. Its fracture is indistinctly conchoidal. Its 
color is dark olive-green and its streak a lighter shade. It is translucent 
or transparent and has a resinous luster. Its hardness =4 and sp. gr. 
= 3.7. Its intermediate refractive index for yellow light is 1.743. 

When heated in the closed tube it yields water and blackens. It is 
easily fusible (2). It yields the usual reaction for Cu and P, and is sol- 
uble in acids and in ammonia. It is distinguished from olivenite by the 
reaction for phosphorus. 

It occurs at many of the localities for olivenite, where, like this min- 
eral, it is a decomposition product of other copper compounds. 



Herderite (CaBe(OH F)P0 4 ) 

Herderite is an isomorphous mixture of the two phosphates, CaBeFP04 
and CaBe(OH)P04. The latter molecule occurs in nature as hydro- 
herderite\ the former occurs only in mixtures. The theoretical compo- 
sition of the fluorine (I) and hydroxyl (II) molecules and of transparent 
crystals from Stoneham (III), and Paris (IV), Maine, are given below: 





BeO 


CuO 


P2O5 


F 


H 2 


Ins. 


Total 
(lessO=F) 


I. 


15-39 


3433 


4353 


11.64 


. * • . 


• • • 




100 


II. 


IS- 53 


34.78 


44.10 


• • • • 


5-59 


• B • 




100 


III. 


i5-5i 


3367 


4374 


S-27 


3-7° 


■ • • 




99.67 


IV. 


16.13 


34 04 


44 05 


5-85 


• • • • 


■44 




100.51 



The mineral is found only in crystals, which are monoclinic, with 
alb: £=.6301 : 1 : .4274 and 18=89° 54'. Their habit is hexagonal, 
pyramidal or short prismatic, elongated in the direction of a. 



PHOSPHATES, ARSENATES AND VANADATES 279 

Herderite is colorless or light yellow, transparent or translucent. 
Its refractive indices are: a= 1.592, 0= 1.612, 7= 1.621. 

Its density is about 3, diminishing, as the amount of hydroxyl in- 
creases, to 2.952 in the pure hydroherderite. 

Before the blowpipe herderite first phosphoresces with an orange- 
yellow light, then fuses to a white enamel, colors the flame red and yields 
fluorine. In the closed glass tube most specimens yield an acid water, 
which, when strongly heated, evolves fluorine that etches the glass. 
The mineral also reacts for phosphorus with magnesium ribbon. It is 
slowly soluble in HC1. 

Occurrence, Origin and Uses. — Herderite occurs in pegmatite dikes 
at Stoneham, Hebron, and other places in Maine, and at the tin mines of 
Ehrenfriedersdorf, Saxony; in all of these places it is apparently of 
pneumatolytic origin. The material from Maine is used to a small 
extent as a gem stone. 

ACID PHOSPHATES 

Acid phosphates are those in which all of the hydrogen atoms of the 
acids have not been replaced by metals or by basic radicals. Theoret- 
ically, they contain replaceable hydrogen atoms. There are 12 or 15 
minerals that are thought to belong to this class, but the composition 
of many of them is very obscure. Most of them appear to be hydrated. 
The only important mineral that may belong to the class is the popular 
gem stone, turquoise. This, according to the best analyses, contains its 
components in the proportions indicated by the formula CuO, 3AI2O3, 
2P2O5, 9H2O, which may be interpreted as (CuOH)(Al(OH) 2 )6H 5 (P04)4. 
which is 4(HaP04), in which 6 hydrogen atoms are replaced by 6A1(0H)2 
groups and one by the group CuOH. 

Turquoise ((CuOH)(Al(OH) 2 )6H 5 (P0 4 )4) 

Turquoise is apparently a definite compound of the formula indicated 
above, which requires 34.12 per cent P2O5, 36.84 per cent AI2O3, 9.57 
per cent CuO and 19.47 per cent H2O. Analysis of a crystallized variety 
from Lynch, Campbell Co., Virginia, gave: 



P205 


A1203 


Fe 2 03 


CuO 


H 2 


Total 

* 


34.13 


36.50 


.21 


9.00 


20.12 


99.96 



Most specimens, however, have not as simple a composition as this. 
They are probably isomorphous mixtures of unidentified phosphates. 



280 DESCRIPTIVE MINERALOGY 

The mineral as usually found is apparently an amorphous or cryp- 
tocrystalline, translucent or opaque material with a waxy luster and a 
sky-blue, green or greenish gray color. Material recently found at 
Lynch, Virginia, however, occurs in minute triclinic crystals with an 
axial ratio .7910 : 1 : .6051, with 0=87° 02', 0=86° 29', and 7=7 2 ° 19'- 
Their habit is pyramidal with 00 Poo (100), 00 P 60 (010), oo'P(iTo), 
ooP'(no) and Poo (0T1). 

The fracture of turquoise is conchoidal. It has a hardness of 5-6 
and a specific gravity between 2.61 and 2.89. It is brittle, and has cleav- 
ages in two directions. The determined refractive indices of the Vir- 
ginia crystals are: a= 1.61, 7= 1.65. 

In the closed tube the mineral decrepitates, yields water and turns 
black or brown. It is infusible, but it assumes a glassy appearance when 
heated before the blowpipe and colors the flame green. When moistened 
with HC1 and again heated the flame is tinged with the azure blue of 
copper chloride. The mineral reacts for copper and phosphoric acid. 
Some specimens dissolve in HC1, but the crystallized material from Vir- 
ginia is insoluble until after it is strongly ignited. It partly dissolves 
in KOH, with the production of a brown residue of a copper compound. 

Occurrence. — Turquoise occurs in thin veins cutting through certain 
decomposed volcanic rocks and other rocks in contact with them, 
and in grains disseminated through them, in stalactites, globular 
masses and crusts. It is probably an alteration product of other com- 
pounds. 

Localities. — Turquoise is found in narrow veins and irregular masses 
in the brecciated portions of acid volcanic rocks and the surrounding clay 
slates, near Nishapur, in Persia; in the Megara Valley, Sinai, and near 
Samarkand, in Turkestan. In all these places the mineral is of gem 
quality and until recently nearly all the gem turquoise came from them. 
Within late years gem turquoise has been discovered in the Cerillo Moun- 
tains, near Santa F6, New Mexico, where it has been mined in consid- 
erable quantity. The locality is the site of an ancient mine which was 
worked by the Mexicans. It is also found and mined in the Burro 
Mountains, Grant County, in the same State, near Millers, and at other 
points in Nevada and near Mineral Park, Mohave County, Arizona, 
where also the ancient Mexicans once had mines. At La Jara, Conejos 
County, Colorado, old mines have likewise been opened up and are now 
yielding gem material. 

Uses. — The only use of turquoise is as a gem stone. Though much 
of the American mineral is pale or green, some of it is of as fine color as 
the Oriental stone. A favorite method of using the stone is in its 



PHOSPHATES, ARSENATES AND VANADATES 281 

matrix. Small pieces of the rock with its included turquoise are pol- 
ished and sold under the name of turquoise matrix. 

Production. — The total value of the turquoise and turquoise matrix 
produced in the United States during 191 1 was $44,751. This weighed 
about 4,363 pounds. In several previous years the production reached 
about $150,000, but in 1912 it was valued at only $10,140. 

HYDROUS PHOSPHATES AND ARSENATES 

HYDRATED NORMAL PHOSPHATES AND ARSENATES 

Of the hydrous salts of orthophosphoric and orthoarsenic acids there 
are two which are of some importance because they are fairly common, 
a third which is utilized in jewelry, and a fourth that is important as an 
indicator of the presence of an ore of cobalt. The first two are vivianite 
and scorodite, a phosphate and an arsenate of iron, the third is variszite, 
an aluminium phosphate, and the fourth is erythrite, an arsenate of 
cobalt. A dimorph of variscite, known as lucinite, is rare. All give 
water in the closed tube and yield phosphine when fused with magne- 
sium and moistened with water. 

VIVIANITE GROUP 

The only important group of the hydrated orthophosphates and 
orthoarsenates is that of which vivianite and erythrite are members. 
The general formula of the group is R" 3 (R'"0 4 ) 2 -8H 2 in which R" 
= Fe, Co, Ni, Zn and Mg, and R"'=P or As. Although some members 
have not been found in measurable crystals, crystals of all have been 
made in the laboratory, so that there is little doubt of their isomorphism. 
All are monoclinic prismatic with axial ratios of about .75 : 1 : .70 and 
P about 74 . The group is as follows: 

Bobierite, Mg 3 (P0 4 ) 2 • 8H 2 Erythrite, C03 (As0 4 ) 2 • 8H 2 

Hornesite, Mg 3 (As0 4 ) 2 ■ 8H2O A nnabergite, Nia (As0 4 ) 2 • 8H 2 

Vivianite, Fe 3 (P0 4 ) 2 • 8H 2 Cabrerite, (Ni • Mg) 3 ( As0 4 ) 2 • 8H 2 

Symplesite, Fe 3 (As0 4 ) 2 • 8H 2 Kottigite, Zn 3 (As0 4 ) 2 • 8H 2 

Only vivianite, erythrite and annabergite are described. 

Vivianite (Fe 3 (P0 4 ) 2 -8H 2 0) 

Vivianite is a common phosphate of iron. It occurs not only in dis- 
tinct crystals but also as bluish green stains on other minerals, and as 
an invisible constituent of certain iron ores, thereby diminishing their 
value. 



282 DESCRIPTIVE MINERALOGY 

Its formula indicates the presence of 43 per cent FeO, 28.3 per cent 
P2O5 and 28.7 per cent H2O. 

Vivianite crystals are monoclinic (prismatic class), usually with a 
prismatic habit. Their axial ratio is .7498 : 1 : .7015, and 18=75° 34'. 
The principal forms observed on them are 00 P 56 (100), 00 P 00 (010), 
oop(no), °oP3(3io), P*(ioi), P(in) and oP(ooi). The angle 
110A 110=71° 58'. The mineral also occurs in stellate groups, in glob- 
ular, fibrous and earthy masses and as crusts coating other compounds. 

Its cleavage is perfect parallel to 00 Pod (010). It is flexible in 
thin splinters and sectile. The fresh, pure mineral is colorless and trans- 
parent, but specimens usually seen are more or less oxidized and have 
a blue or green color. It has a vitreous to pearly luster. Its streak is 
white or bluish, changing to indigo-blue or brown on exposure to the air. 
Its pleochroism is strong in blue and pale yellow tints. Its hardness 
is 1.5-2 and density about 2.6. Its refractive indices for yellow light 
are: a= 1. 5818, 0=1.6012, 7=1.6360. 

In the closed tube vivianite whitens, exfoliates and yields water at a 
low temperature. It fuses easily (2), tingeing the flame bluish green. 
Its fusion temperature is n 14°. The fused mass forms a grayish black 
magnetic globule. It gives the reaction for iron, and is soluble in HC1. 

The mineral is easily recognized by its softness, easy fusibility and 
by yielding the test for phosphorus. 

Synthesis. — Crystals have been made by heating iron phosphate with 
a great excess of sodium phosphate for eight days. 

Occurrence and Origin. — Vivianite occurs in veins of copper, tin and 
gold ores; disseminated through peat, clay, and limonite; coating the 
walls of clefts in feldspars and other minerals of certain igneous rocks, 
and partially filling cavities in fossils and partly fossilized bones. It is 
usually the result of the decomposition of other minerals. 

Localities. — Crystals are found at several points in Cornwall, Eng- 
land; at the gold mines at Verespatak, in Transylvania; at Allentown, 
Monmouth County, New Jersey, and at many other places. The earthy 
variety occurs at Allentown, Mullica Hill and other points in New Jer- 
sey, in Stafford County, Virginia, and in swamp deposits at many places. 
It is abundant in limonite at Vaudreuil, in Quebec, and in bog iron ores 
elsewhere. 

Erythrite (Co 3 (As0 4 )2-8H 2 0) 

Erythrite, or cobalt bloom, is not a common mineral, but, because 
of its beauty and the fact that it is the usual alteration product of cobalt 
ores, it deserves to be described. 



PHOSPHATES, ARSENATES AND VANADATES 283 

In composition erythrite is 37.5 per cent CoO, 38.4 per cent AS2O5, 
and 24.1 per cent H2O. It usually, however, contains some iron, nickel 
and calcium. 

The mineral is isomorphous with vivianite. Its crystals are mono- 
clinic and prismatic or acicular and their axial ratio is .7937 : 1 : .7356 
and 0=74° 51'. The prisms are striated vertically. Erythrite occurs 
in all the forms in which vivianite is found. Its crystals are usually 
bounded by 00 P 00 (010), ooP(no), 00 Poo (100), +P<x>(Toi) and 

P(Tn). 

The cleavage of erythrite is perfect parallel to 00 P 00 (010). It is 

transparent or translucent, has a gray, crimson or peach-red color, 

and a white or pink streak. Its hardness varies between 1.5 and 2.5 

and its density is 2.95. Its luster is pearly on 00 P 00 (010) and 

vitreous on other faces. It is flexible and sectile. Its refractive 

indices for yellow light are: a= 1.6263, 0= 1.6614, 7= 1.6986. 

In the closed tube erythrite turns blue and yields water at a low tem- 
perature. At a high temperature it yields AS2O3, which condenses in 
the cold portion of the tube as a dark sublimate. It fuses at 2, and 
tinges the flame pale blue. On charcoal it fuses, yields arsenic fumes and 
a gray globule which colors the borax bead a deep blue. The mineral 
is soluble in HC1, giving rise to a pink solution, which, upon evaporation 
to dryness, gives a blue stain. 

It is easily recognized by its color and the cobalt reaction. It is 
readily distinguished from pink tourma.ine (p. 434), by its hardness 
and easy fusibility. 

Synthesis. — Crystals have been obtained by carefully mixing to- 
gether warm solutions of C0SO4 and HNa2As04 • 7H2O. 

Occurrence. — Erythrite occurs in the upper portions of veins con- 
taining cobalt minerals, being formed by their weathering. 

Localities. — It occurs as scales and crystals at Schneeberg, Saxony, 
and as crystals at Modum, Norway. It is found, also, at Lovelock's 
Station, Nevada, at several points in California and in large quantities 
at Cobalt, Ontario. 

Annabergite (Ni3(As0 4 )2-8H 2 0) 

Annabergite, or nickel bloom, is isomorphous with erythrite. It 
occurs massive, disseminated in tiny grains through certain rocks, as 
crusts and stains in globular and earthy masses, and in fibrous crystals, 
the axial ratios of which are not known. 

The mineral is apple-green in color, and is translucent or opaque. 



284 DESCRIPTIVE MINERALOGY 

Its streak is light green. Its luster is vitreous; its hardness, 1.5-2.5 
and sp. gr. = 3. 

Before the blowpipe it melts to a gray globule and gives the arsenic 
odor. In the closed glass tube it blackens and yields water. In the 
beads it gives the usual reactions for Ni. The mineral dissolves easily 
in acids. 

Synthesis. — Crystals have been produced by the method employed 
in the synthesis of erythrite, using NiSQt, instead of C0SO4. 

Occurrence. — It is found as a common alteration product of nickel- 
bearing minerals, in the oxidized portions of veins. 

Localitus. — Its best known occurrences are in Allemont, Dauphin^; 
Annaberg and Schneeberg, Saxony; Cobalt, Ontario, and mines in 
Colorado and Nevada. 



Variscite (AIPO42H2O) 

Variscite is a bright green mineral that has recently come into use as 
a gem material. It is apparently an aluminium phosphate with a 
theoretical composition as follows: 44.9 per cent P2O5; 32.3 per cent 
AI2O3 and 22.8 per cent H2O. A specimen of crystallized material from 
Lucin, Utah, gave the following analysis: 



P205 


A1203 


Fe203 


Cr0 3 


V2O3 


H 2 


Total 


44.73 


32-40 


.06 


.18 


•3 2 


22.68 


100.37 



Recent investigations indicate that the compound A1P04-2H20 is 
dimorphous. Both forms are orthorhombic but one, variscite, has the 
properties described under this heading. The other, lucinite, is associ- 
ated with variscite, near Lucin, Utah. It, however, occurs in crystals 
that are octahedral in habit, rather than tabular, and that have an 
axial ratio of .8729 : 1 : .9788. In other respects lucinite is very much 
like variscite. 

An amorphous variety of the same substance is also kaiown. It 
occurs as a white, pale brown or pale blue earthy mass with a sp. gr. of 
2.135. I* differs from the crystalline varieties in being completely 
soluble in warm concentrated H2SO4. 

The crystals of variscite are orthorhombic and are bounded by 
00 P 60 (010), 00 P(no) and ^P 06 (012), and in a few cases 00 P 60 (100). 
Their axial ratio is .8944 : 1 : 1.0919. Nearly all crystals are tabular 
parallel to 00 P 60 (010). Twins are common, with ^P 60 (102) the 
twinning plane. » Crystals are comparatively rare, the mineral occur- 
ring usually in fibrous or finely granular masses and as incrustations. 



PHOSPHATES, ARSENATES AND VANADATES 285 

Variscite varies in color from a pale to a bright green. It is weakly 
pleochroic, has a vitreous luster, a hardness of about 4 and a density of 
2.54. Its refractive indices for yellow light are: a= 1.546, 18=1.556, 

y= 1-578- 

Before the blowpipe the mineral is infusible. It, however, whitens 
and colors the flame deep bluish green. It yields water in the closed 
tube, and with the loss of its water, it changes color from green to 
lavender. The same change in color takes place gradually at temper- 
atures between iio°-i6o°. When heated with Co(N03)2, it turns blue 
and when fused with magnesium ribbon it gives the test for phosphorus. 
It forms a yellowish green glass with borax or microcosmic salt. The 
mineral is insoluble in acids before heating. 

Variscite resembles in some respects certain varieties of turquoise 
and wavelliie (p. 287). It is distinguished from turquoise by the absence 
of copper and from wavellite by its insolubility in acids. 

Occurrence. — The mineral occurs as a cement in a brecciated, cherty 
limestone and a brecciated rhyolite, as nodules in the cherty portions 
of the breccias and also as veins traversing these rocks. It is also 
found as nests in weathered pegmatites. The crystals occur as coarsely 
granular, loosely coherent masses in more compact granular masses. 

Localities. — Variscite occurs at Messbach, Saxony; in Montgomery 
County, Arkansas; near Lucin, Utah, and at a number of other places 
in Tooele and Washington Counties in this State; in Esmeralda County, 
Nevada, and in Montgomery County, Arkansas. The colloidal variety 
occurs as concretions in slates at Brandberg, near Leoben, Austria. 

Uses. — The mixture of variscite and rock is cut, and employed as 
sets in necklaces, belt pins, etc., under the names " utahlite " and 
" amatrice," but because of the softness of the variscite it cannot be 
used with success for all the purposes for which turquoise matrix is 
used. 

Production. — The production of the material in the United States 
during 191 1 was 540 lb., valued at $5,750. In the previous year 
5,377 lb. were reported as having been sold for $26,125. In 191 2, 
the amount marketed was valued at $8,150. 

Skorodite (FeAs(V 2H2O) 

Skorodite is more common than vivianite. It occurs in globular 
and earthy masses, as incrustations, and in crystals of a green or brown 
color. The globular forms are colloidal. 

Its formula indicates Fe2C>3 = 34.6 per cent, As203 = 49-8 per cent 



286 



DESCRIPTIVE MINERALOGY 



and HaO= 15.6 per cent. An incrustation on the deposits of the Joseph's 
Coat Spring, Yellowstone National Park, consisted of: 



As 2 5 


Fe203 


H 2 


Si02 


SO3 


Total 


46.48 


33 29 


15.50 


435 


.84 


100.46 




Its crystallization is orthorhombic (bipyramidal class), with a : b : c 
= .8658 : 1 : .9541. The crystals, which are commonly bounded by 

00 P 56 (lOo), 00 P06 (oio), 00P2(l2o), OOP(HO), 

P(iii) and £P(ii2), are either prismatic or octa- 
hedral in habit (Fig. 159). The angle 111A1I1 

= 65° 20'. Their cleavage is imperfect, parallel to 

ooP(no). 

The mineral is brittle. It has a vitreous luster, 
a leek-green or liver-brown color and a white 
streak. It is translucent and has an uneven frac- 
ture. Its hardness is 3.5-4 and density about 3.3. 

Fig. 150. — Skorodite ^he colloidal phases are somewhat softer than the 
Crystal with 00 P 00 , . ... , 

, . ~- crystalline phases. 

100 (a); 00 P2, 120 J 11 

(d), and P, in (p) * n tne c * ose d tube skorodite turns yellow and 

yields water. It fuses easily, coloring the flame 
bluish. On charcoal it yields white arsenical fumes and gives a black 
porous, magnetic button. It is soluble in HC1, forming a brown solution. 

It is distinguished from livianite by the arsenic test, and from dufren- 
ite by its streak and reaction in the closed tube. 

Synthesis. — Skorodite crystals have been made by heating metallic 
iron with concentrated arsenic acid solution at i40°-i5o°. 

Occurrence. — Skorodite is frequently associated with arsenopyrite, 
in the oxidized portions of veins containing iron minerals. It is found 
also in a few places as incrustations deposited by hot springs. 

Localities. — It occurs in fine crystals at Nerchinsk, Siberia; at 
Loelling, in Carinthia; near Edenville, New York; in the Tintic dis- 
trict, Utah, and as an incrustation on the siliceous sinter of the geysers 
in Yellowstone Park. 



HYDRATED BASIC PHOSPHATES AND ARSENATES 

The hydrated basic phosphates and arsenates are rather more nu- 
merous than the hydrated normal compounds, but most of them are rare. 
One, wavellite, however, is a handsome mineral that is fairly common. 
Another, pkarmacosiderite, an iron arsenate, is known to occur at a 
number of places. The uranite group also belongs here. Its members 



PHOSPHATES, ARSENATES AND VANADATES 287 

are comparatively rare, but, because of the presence of uranium in them, 
they are of considerable interest. 

WaveUite ((Al(OH-F) 3 )(P04)a-5HsO) 
Wavellite rarely occurs in crystals. It is usually in acicular aggre- 
gates that are either globular or radiating (Fig. 160). The few crystals 
that have been seen are orthorhombic (bipyramidal class), with an 
axial ratio of .5573 : 1 : .4057. 

Its composition varies widely, and frequently a fairly large portion 
of the OH is replaced by F, and a portion of the Al by Fe. 

The mineral is vitreous in luster and white, green, yellow, brown or 
black in color. Its streak is white. It is brittle and translucent, in- 



Fig. 160.— Radiate Wavellile on a Rock Surface. 

fusible and insoluble in acids. Its hardness is 3.5 and its density 3.41. 
Its intermediate refractive index for yellow light is 1.526. 

Heated in a dosed glass tube, wavellite yields water, the last traces 
of which react acid and often etch the glass. In the blowpipe flame the 
mineral swells up and breaks into tiny infusible fragments, at the same 
time tingeing the flame green. The mineral is soluble in HC1 and 
H2SO4. When heated with H2SO4 many specimens yield hydrofluoric 
acid. When heated on charcoal and moistened with Co(N03)2 and 
reheated, the mineral turns blue. 

Wavellite is distinguished from turquoise, which it sometimes 
resembles, by its action in the blowpipe flame, by its inferior hardness 
and its manner of occurrence. 

Occurrence- — Wavellite occurs as radiating bundles on the walls of 



288 



DESCRIPTIVE MINERALOGY 



cracks in various rocks and as globular masses filling ore veins and the 
spaces between the fragments of breccias. It is probably in all cases 
the result of weathering. 

Localities. — It is found at a great number of places, especially at 
Zbirow, in Bohemia; at Minas Geraes, Brazil; at Magnet Cove, Arkan- 
sas, and in the slate quarries in York County, Penn. 

Pharmacosiderite ((FeOH) 3 (As0 4 ) 2 5H2O) 

Pharmacosiderite is a hydrated ferric arsenate, the composition of 
which is not firmly established. It usually occurs in small isometric 
crystals (hextetrahedral class), that are commonly combinations of 

00 O 00 (100) and —(in). It is also sometimes found in granular 

2 

masses. Its cleavage is parallel to 00 O 00 (100). 

The mineral is green, dark brown or yellow. Its streak is a pale 
shade of the same color. It has an adamantine luster and is translucent. 
Its hardness = 2.5 and sp. gr. = 3. It is sectile and pyroelectric. Its 
refractive index, n= 1.676. 

Pharmacosiderite reacts like skorodite before the blowpipe and with 
reagents. 

The mineral occurs in the oxidized portions of ore veins, in Cornwall, 
England; at Schneeberg, Saxony; near Schemnitz, Hungary; and in the 
Tintic district, Utah. 

URANITE GROUP 

The uranites are a group of phosphates, arsenates and vanadates 
containing uranium in the form of the radical uranyl (UO2) which is 
bivalent. The members of the group are either tetragonal, or ortho- 
rhombic with a tetragonal habit. They all contain eight molecules of 
water of crystallization. Only three members of the group are of 
sufficient interest to be discussed here. These are the hydrated cop- 
per and calcium uranyl phosphates, torbernite and autunite and the 
potassium uranyl vanadate, carnotite. 

The entire group so far as its members have been identified is as 
follows: 



Autunite 

Uranospinite 

Torbernite 

Zeunerite 

Uranocircite 

Carnotite 



Ca(U0 2 )2(P04)2-8H 2 

Ca(U0 2 )2(As04)2-8H 2 

Cu(U02)2(P0 4 )2-8H 2 

Cu(U0 2 )2(As04)2-8H 2 

Ba(U0 2 )2(P04)2-8H 2 

(Ca • K 2 ) (U0 2 )2(V0 4 )2 • xHaO 



Orthorhombic 

Orthorhombic 

Tetragonal 

Tetragonal 

Orthorhombic 



k 



PHOSPHATES, ARSENATES AND VANADATES 289 

The uranites are of interest because of their content of uranium, an 
element which is genetically related to radium. 

Autunite (Ca(U02)2(P0 4 )2-8H 2 0) 

Autunite occurs in thin tabular crystals with a distinctly tetragonal 
habit, and in foliated and micaceous masses. 

The percentage composition corresponding to the above formula 
is 6.i per cent CaO, 62.7 per cent UO3, 15.5 per cent P2O5 and 15.7 per 
cent H2O. 

Its crystals are orthorhombic (bipyramidal class), with an axial 
ratio, .9875 : 1 : 2.8517, thus possessing interfacial angles that closely 
approach those of torbernite. Its crystals are bounded by oP(ooi), 
Poo (101), P06 (on), and several less prominent planes. Their cleav- 
age is very perfect and the cleavage lamellae are brittle. The luster is 
pearly on the base and vitreous on other surfaces. 

The mineral is lemon-yellow or sulphur-yellow in color, and its streak 
is yellow. It is transparent to translucent. Its hardness is 2-2.5 an d 
its specific gravity about 3.2. Its refractive indices for yellow light are: 

« = i.553> 0=i-575> 7=1.577- 

The mineral reacts like torbernite before the blowpipe and with acids, 

except that it shows none of the tests for copper. It is recognized by its 

color, streak and specific gravity. 

Occurrence. — Autunite occurs in pegmatite veins and on the walls 
of cracks in rocks near igneous intrusions, especially in association with 
other uranium compounds, of which it is a decomposition product. 

Localities. — It has been found at Johanngeorgenstadt, Germany; 
at Middletown and Branch ville, Conn., in the mica mines of Mitchell 
County, North Carolina, and coating cracks in gneiss at Baltimore, Md. 

Torbernite (Cu(U0 2 )2(P0 4 )2 -8H 2 0) 

Torbernite occurs in small square tables, that may be very thin or 
moderately thick, and in foliated and micaceous masses. 

The pure mineral contains 61.2 per cent UO3, 8.4 per cent Cu, 
15.1 per cent P2O5 and 15.3 per cent H2O, but frequently a part of the P 
is replaced by As. 

Its crystals are tetragonal (ditetragonal bipyramidal class), with 
a : c=i : 2.9361. They are extremely simple, their predominating 
forms being oP(ooi) and Poo (101). Less prominent are 00 Poo (100), 
2Poo(2oi) and ooP(no). Their cleavage is perfect parallel to oP. 
The cleavage lamellae may be almost as thin as those of the micas 
but they are brittle. 



290 DESCRIPTIVE MINERALOGY 

The mineral is bright green in emerald, grass or apple shades, has a 
lighter green streak, is translucent or transparent, and has a hardness 
of 2.25 and a specific gravity of about 3.5. Its luster is pearly on the 
basal plane, but nearly vitreous on other surfaces. It is strongly pleo- 
chroic in green and blue. 

Torbernite gives reactions for Cu and P and yields water in the 
closed tube. The bead reactions for uranium are masked by those of 
copper. The mineral is soluble in HNO3. 

The mineral is easily recognized by its color and other physical 
properties. 

Occurrence. — Torbernite is occasionally found as a coating on the 
walls of crevices in rocks. It occurs in Cornwall, England; at Schee- 
berg, Saxony; at Joachimsthal, Bohemia, and at most places where other 
uranium minerals exist. It is probably in all cases a weathering product. 

Carnotite ((Ca-K 2 )(U0 2 ) 2 (V0 4 )2-xH20) 

Carnotite, like the other uranites described, is extremely complex 
in composition. It may be an impure potassium uranyl vanadate, or a 
mixture of several vanadates in which the potassium uranyl compound 
is the most prominent. The formula given above indicates its com- 
position as well as any simple formula that has been proposed. A 
specimen from La Sal Creek, Colorado, shows the mineral to be essen- 
tially as follows: 

« 

V2O5 U0 3 CaO BaO K 2 H 2 at 105 ° H 2 above 105 ° 
18.05 54 - 00 I -^° J -86 5 46 3.16 2.21 

though there are present in the specimen analyzed, or in other specimens 
from the same locality, also As 2 03, P 2 Os, Si0 2 , Ti0 2 , C0 2 , SO3, M0O3, 
Cr 2 3 , Fe 2 3 , A1 2 3 , PbO, CuO, SrO, MgO, Li 2 and Na 2 0, and there 
are reported in them also small quantities of radium. Radiographs 
taken with the aid of carnotite have been published, which are almost 
as clear as those taken with pitchblende. The complete analysis of a 
specimen from the Copper Prince Claim, Montrose Co., Colo., gave: 



v 2 o 5 


As 2 05 


P 2 5 


UO3 


M0O3 


Fe 2 03 


Al 2 0s PbO 


18.35 


25 


•33 


5 22 5 


• 2 3 


1.77 


1.08 .25 


CuO 


CaO 


BaO 


K 2 


H 2 0- 


H 2 0+ 


Ins. Total 


.20 


2.85 


.72 


6-73 


2-59 


3.06 


8.34 99.84 



Also Ti0 2 = .io; C0 2 = 33; S03 = .i2; Cr03 = tr.; MgO=.2o and 
Na 2 0=.o9. 



PHOSPHATES, ARSENATES AND VANADATES 291 

The mineral has been found only in tiny crystalline grains, so that its 
physical properties are not well known. It is bright yellow in color, and 
is completely soluble in HNO3. If to the nitric acid solution hydro- 
gen peroxide be added a brown color will appear. Or if the solution 
is filtered, made alkaline by ammonia and through it is passed H2S, a 
garnet color will develop. If the mineral be moistened by a drop of 
concentrated HC1, a rich brown color will result. The addition of a drop 
or two of water will change the color to light green or make it disappear. 

Occurrence. — Carnotite occurs as a yellow crystalline powder, some 
of which seems to consist of minute crystals with an hexagonal habit, 
in the interstices between the grains in sandstones and conglomer- 
ates, as nodules or lumps in these rocks, and as coatings on the walls 
of cracks in pebbles in the conglomerates and on pieces of silicified 
wood embedded in the sandstones. It is limited to very shallow 
depths and is apparently a deposit from ground water. 

Localities. — Its principal known occurrences are in Montrose, San 
Miguel, Mesa and Dolores Counties in southwestern Colorado, especially 
in Paradox Valley, and in adjoining portions of New Mexico and Utah, 
and in Rio Blanco and Routt Counties in the northwestern portion of 
Colorado. At all these places there are large quantities of the impreg- 
nated rock but it contains on the average only about 1.5 per cent to 
2 per cent of U3O8. The mineral has also been described from Mt. 
Pisgah, Mauch Chunk, Pennsylvania, and from Radium Hill, South 
Australia. 

Uses. — The mineral is one of the main sources of radium and uranium 
and is one of the principal sources of vanadium. Although it contains a 
notable quantity of uranium, carnotite has little value except as an ore 
of radium and vanadium, because of the few uses to which uranium is 
put. This metal is used to some extent in making steel alloys and in the 
manufacture of iridescent glazes and glass. Its compounds are used in 
certain chemical determinations, as medicines, in photography, as por- 
celain paint, and as a dye in calico printing. The uses of vanadium have 
been referred to on p. 273. 

The principal value of carnotite depends upon its content of radium, 
which in the form of the chloride is valued at about $40,000 per gram 
or $1,500,000 per oz. The importance of radium as a therapeutic agent 
has not been established; but that its use is wonderfully helpful in many 
diseases is beyond question. Without doubt in the near future carno- 
tite will become the principal source of radium in the world. Practically 
the only other source is the pitchblende (p. 297), of Gilpin, Colorado, 
Cornwall, England and Joachimsthal, Austria, 



292 DESCRIPTIVE MINERALOGY 

Production. — Carnotite has been mined in San Miguel and Montrose 
Counties, Colorado, and at several points in eastern Utah, but mainly 
for the vanadium it contains. At present it is being utilized as a source 
of radium. From Colorado 8,400 tons of vanadium ore, with a value 
of $302,000, were shipped in 19 11 and from New Mexico and Utah about 
70 tons, valued at $3,500. Some of this, however, was vanadinite. 
Most of it was exported and used as a source of vanadium. However, 
the uranium content of the carnotite mined was about 1.1 tons of the 
metal. During 191 2 ore containing 26 tons of uranium oxide and 6.7 
grams of radium was produced. This would have yielded n.43 grams 
of radium bromide, valued at $52,800. The present price of standard 
carnotite carrying at least 2 per cent U3O8 and 5 per cent V2O5, is at the 
rate of $1.25 per lb. for the former and thirty cents for the latter. In 
1914 the selling price of 4,294 tons of carnotite ore containing 87 tons 
of U3O8 was $103 per ton. At the present time nothing is paid for the 
radium content of the ore, though this is its most valuable component. 
One ton of ore containing 1 per cent of U3O8 carries 2.566 milligrams of 
radium. The imports of uranium compounds during 191 2 were valued 

at $14,357- 

HYDRATED ACID PHOSPHATES AND ARSENATES 

A number of hydrated acid phosphates and arsenates are known to 
constitute an isomorphous group, but only a few of them occur as 
minerals. Brushiie is an acid calcium phosphate and pharmacolite is 
the corresponding arsenate. Both crystallize in the monoclinic system 
(prismatic class). Neither is common. 

Pharmacolite (HCaAs0 4 • 2H2O) occurs principally in silky fibers, in 
botryoidal and stalactic masses and rarely in crystals with an axial 
ratio .6236 : 1 : .3548 and #=83° 13'. Their cleavage is perfect par- 
allel to 00 Poo (010). The mineral is white or gray, tinged with red. 
Its streak is white. It is translucent or opaque. Its luster is vitreous, 
except on 00 P 00 (dio) where it is slightly pearly. Thin laminae are 
flexible. Its hardness is 2-2.5 an d density 2.7. Its refractive indices 
for yellow light are: a= 1.5825, 18=1.5891, 7=1.5937. 

Before the blowpipe pharmacolite swells up and melts to a white 
enamel. The mineral gives the usual reactions for As, H2O and Ca. It 
usually occurs in the weathered zone of arsenical ores of Fe, Ag and Co, 
at Andreasberg, Harz; Joachimsthal, Bohemia, and elsewhere. 



CHAPTER XV 

THE COLUMBATES, TANTALATES AND URANATES 

The rare metals, columbium and tantalum, exist in a few silicates, 
but their principal occurrences are as columbates and tantalates which 
are salts of columbium and tantalum acids, analogous to the various 
acids of sulphur. The commonest compounds are salts of the meta- 
acids IfeQ^Oe and H2Ta20e, the relations of which, to the normal acids, 
are indicated by the equation 2H3CbC>4— 2H20=H2Cb20e. Other im- 
portant minerals are derivatives of the pyroacids corresponding to 
H4Ct>207, or 2H3CDO4— H2O. The best known ortho salt is ferguson- 
ite, YCb04, but it is rare. 

All the columbates yield a blue solution when partially decomposed 
in H2SO4 and boiled with HC1 and metallic tin. The tantalates when 
fused with KHSO4 and treated with dilute HC1 give a yellow solution 
and a heavy white precipitate, which, on treatment with metallic zinc 
or tin, assumes a deep blue color. When diluted with water the blue 
color of the tantalate solution disappears, while that of the columbate 
solution remains. 

The uranates are salts of uranic acid, H2UO4. The only mineral 
known that may be a uranate is uraninite, and the composition of this 
is doubtful. 

Columbite ((Fe-Mn)Nb 2 6 ) and Tantalite ((FeMn)Ta 2 6 ) 

These two minerals are isomorphous mixtures of iron and manganese 
columbates and tantalates. The name columbite is applied to the mix- 
ture that is composed mainly of the columbates, and tantalite to that 
which is principally a mixture of tantalates. When the tantalite is 
composed almost exclusively of the manganese molecule, it is known as 
manganotantalitei Tin and tungsten are frequently found in both min- 
erals. 

Their crystals are orthorhombic, with a : b : £=.8285 : 1 : .8898 for 
the nearly pure columbium compound, and .8304 : 1 : .8732 for the 
nearly pure tantalum compound. Both form short prismatic crystals 
containing many faces, among the most prominent being the three 
pinacoids; various prisms, notably 00 P(no), 00 P^^o) and 00 P6(i6o), 

293 



294 



DESCRIPTIVE MINERALOGY 



and the domes 2P &> {201) and JP 06 (012) (Fig. 161). The most promi- 
nent pyramids are P(in) and P3(i33)- Twins are not uncommon, 
with 2P00 (201) the twinning plane. The angle 110A1T0 for colum- 
bite=79° 17'. 

Both minerals are usually opaque, black and lustrous, and occasion- 
ally iridescent, though, in some instances, they are translucent and 
brown. Their streak is dark red or black. Their cleavage is distinct 
parallel to 00 P 66 (100), fracture uneven or conchoidal, their hardness 
6 and their specific gravity 
between 5.3 and 7.3, in- 
creasing with the propor- 
tion of the tantalum mole- 
cules present. They are 
both infusible before the 
blowpipe. Some specimens 
exhibit weak radioactivity. 
When columbite is de- 
composed by fusion with 
KOH and dissolved in HC1 
and H2SO4, the solution 
1 turns blue on the addition 
of metallic zinc. The min- 
eral is also partially decom- 
posed when evaporated to 
dryness with H2SO4, forming a white compound that changes to yellow. 
When this residue is boiled with HCI and metallic zinc a blue solution 
results. The mineral also gives reactions for iron and manganese. 

Tantalite is decomposed upon fusion with KHSO4 in a platinum 
spoon, or on foil. This when heated with dilute HCI yields a yellow 
solution and a heavy white powder. Upon addition of metallic zinc, a 
blue color results and this disappears on dilution with water. In the 
microcosmic salt bead tantalite dissolves slowly, giving reactions for iron 
and manganese. When treated with tin on charcoal the bead turns 
green. 

The two minerals may easily be confused with black tourmaline 
(p. 434), ilmenite (p. 462) and wolframite. From tourmaline, they are 
distinguished by crystallization, high specific gravity and luster; from 
wolframite by their less perfect cleavage and by the reaction with 
aqua regia (see p. 259); from ilmenite by the test for titanium, 

Occurrence, Origin and Localities. — Both minerals occur in veins of 
coarse granite and probably have a pneumatolytic origin. 



Fits. 



1 .—Columbite Crystals with «P«, 1 
«=P«,o.o (6); «.p, iio(m); "Pi.? 
° p l . 73° W); »P3, 130 is); iP«=, 1 
P, in (o) and P3, 133 («)■ 



COLUMBATES, TANTALATES AND URANATES 295 

i 

Columbite is found in granite veins at Bodenmais, Bavaria; Tam- 
mela, in Finland; near Limoges, France, with tantalite; near Miask, 
in the Ilmen Mountains, Russia, with samarskite; and at Ivigtut, in 
Greenland. In the United States it is found at Standish and Stone- 
ham, in Maine; at Acworth, in New Hampshire; at Haddam, in Con- 
necticut; at Amelia Court House, Virginia; with samarskite in the mica 
mines in Mitchell County, North Carolina; in the Black Hills, South 
Dakota, and at a number of other points in New England and the Far 
West. 

Tantalite is found at many of the localities for columbite and also 
at several other places in Finland; near Falun, in Sweden; in Yancy 
County, North Carolina, and in Coosa County, Alabama. 

Uses. — At the present time columbium and its compounds have no 
commercial uses. Tantalum, however, is employed in the manufacture 
of filaments for certain types of incandescent lamps. Since, however, 
about 20,000 filaments may be made from a single pound of the metal the 
market for tantalum ores is very limited. 

Samarskite and Yttrotantalite 

These two minerals may be regarded as isomorphous mixtures of 
salts of pyrocolumbic and pyrotantalic acids,, in which the bases are 
yttrium, iron, calcium and uranyl. 

Samarskite, according to this view, is approximately 

Y2(Ca-Fe.U02)3(Nb 2 7 )3 

and yttrotantalite the corresponding tantalate. Yttrium and iron are 
the principal bases, but there are also often present erbium, cerium, 
tungsten and tin. 

Analyses made by Rammelsberg and quoted by Dana give some idea 
of the complexity of the compounds: 



Density 


Ta 2 5 Nb 2 5 W0 3 Sn0 2 Ti0 2 * Y2O3 


Er 2 03 


I- 5-4*5 


46.25 12.32 2.36 1. 12 10.52 


6.71 


n. 5.839 


14.36 41.07 .16 .56 6.10 


10.80 


in. 5.672 


55 34 .22 1.08 8.80 


382 


Ce 2 03t 


U0 2 FeO CaO H2O 


Total 


I. 2.22 


1. 61 3.80 5.73 6.31 


98-95 


II. 2.37 


10.90 14.61 .... 


100.93 


ni. 433 


11.94 1430 


99 83 


1. From Itterby, Sweden. II. From North Carolina. IH. From Miask 


. Russia. 


* 


Including Si02. t Including Di 3 3 and LajOj. 





296 DESCRIPTIVE MINERALOGY 

The first of these three minerals has been called yttrotantalite and 
the other two samarskite. If the first is weathered, as seems probable 
from the presence of over six per cent of water, the three may constitute 
members of an isomorphous series with the third representing the nearly 
pure columbate (samarskite), the first a compound in which the tantalate 
molecule is in excess (yttrotantalite), and the second an intermediate 
compound which contains both the tantalum and columbium molecules, 
with the latter predominating. 

With more accurate analyses the great complexity of these compounds 
becomes even more apparent. Hillebrand has given the following report 
of his analysis of a samarskite from Devil's Head Mountain, near Pike's 
Peak, Colorado, which shows the futility of attempting to represent its 
composition by a chemical formula: 

Pitch-black Black Weathered 

Variety Variety Variety 

Tajd 27.03 28.II 19-34 

Cb 2 Oft 27.77 26.16 27.56 

WOi 2.25 2.08 5.51 

SnO« 95 1.09 .82 

ZrOj 2 . 29 2 . 60 3 . 10* 

UO» 4.02 4.22 

UOj ... 6 . 20 

ThO* 3.64 3.60 3.19 

Ce 8 8 54 .49 .41 

(La,Di) 2 8 1.80 2.12 1.44 

Erj0 2 10.71 10.70 9.82 

Y2O3 6.41 5.96 5.64 

Fe*0, 8.77 8.72 8.90 

FeO 32 .35 . 39 t 

MnO 78 .75 1 

ZnO 05 .07 I' 77 

PbO 72 .80 1.07 

CaO 27 .33 1. 61 

MgO ... .11 

K *° 17 .13 \ , 

(Na,Li),0 24 .17 /' 3 

H2O 1.58 1.30 3.94 

F ? ? ? 

Total 100.31 99-75 100.18 

Sp.gr 6.18 6.12 5.45 

♦With Ti0 2 . t Or 0.74 UO*. 



COLUMBATES, TANTALATES AND UEANATES 297 




Both samarskite and yttrotantalite are orthorhombic, with an axial 
ratio for samarskite of .5456 : 1 : .5178, and for yttrotantalite, .5411 : 
1 : 1. 1330. They, however, more commonly occur massive and in 
flattened grains embedded in rocks. Their crystals are prismatic in 
the direction of the c or the b axis. Their most prominent forms are 
00 P* (100), 00 P 66 (010) and P 66(101) (Fig. 162). Less prominent 
but fairly common are *>P2(i2o), ooP(no), P(iu) and 3Pf(23i). 
The angle 110A1T0 for samarskite is 57 14' 
and for yttrotantalite 56 50'. 

The cleavage of both minerals is indistinct 
parallel to 00 P 06 (010). Their fracture is 
conchoidal. Both are brittle. The hardness of 
samarskite is 5-6, its density about 5.7, its 
luster vitreous, its color velvety black and its 
streak reddish brown. Yttrotantalite is a little 
softer (5-5.5). Its specific gravity is 5.5-5.9, 
its luster submetallic to vitreous, its color black, Fig. 162.— Samarskite Crys- 
brown, or yellow, and its streak gray to color- **! with °° P * , 100 (a); 
less. Samarskite is opaque and yttrotantalite 
opaque or translucent. 

The reactions of the minerals vary with 
their composition. They always yield the 
blue solution test for tantalum or columbium, and most specimens react 
for Mn, Fe, Ti and U. The reaction for uranium is an emerald green 
bead with microcosmic salt in both reducing and oxidizing flame. 

They are distinguished from columbite and tantalite by the form of 
their crystals. 

Occurrence. — The two minerals, like columbite and tantalite, are 
found principally in pegmatite veins and in many of the same localities. 
Yttrotantalite occurs mainly at Ytterby and near Falun, in Sweden, and 
samarskite, near Miask in the Ilmen Mountains, Russia. In the United 
States the last-named mineral is sometimes found in large masses in the 
mica pegmatites of Mitchell County, North Carolina. 

Uses. — Neither mineral is at present of any commercial value. They 
are, however, extremely interesting as the source of many of the rare 
elements, and, especially, as a possible source of radium and closely 
related substances. 

Uraninite 

Uraninite, or pitchblende, like the other compounds containing the 
element uranium, is of doubtful composition. It contains so many 



00 Poo, 010 (b); 00 p, 
no (m); 00 P2, 120 (A); 
P«, 101 («); P,ni (p); 
3 Pf, 231 (»). 



298 DESCRIPTIVE MINERALOGY 

different components that a correct conception of its character is almost 
impossible to grasp. The mineral is particularly interesting because it 
always contains a trace of radium, of which it is an important com- 
mercial source at the present time. 

Analyses of crystallized material (I) from Branchville, Conn., 
and from Annerod (II), Norway, gave the following results: 

U0 3 U0 2 TI1O2 PbO FejjOa CaO H 2 He Insol. 

I. 21.54 64.72 6.93 4.34 .28 .22 .67 Und. .14 

II.30.63 46.13 6.00 9.04 .25 .37 .74 .17 442 

with small quantities also of Z1O2, CeCfe, La203, Di203, Y2O3, Er2C>3, 
MnO, Alkalies, Si02 and P2O5. These analyses are interpreted as indi- 
cating that the mineral is a uranium salt of uranic acid, UQ2(OH)2, or 

H2UO4, thus U -^> n , or U3O8, in which Pb replaces the U in 

\)> U02 
part, and TI1O2 the UO2. Radium is found in most specimens and 
helium in nearly all. 

Several varieties are recognized, the distinctions being based largely 
upon chemical differences. 

Broggerite has UO3 to other bases as 1 : 1. 

Cleveite and nhenile contain 9 per cent to 10 per cent of the yttria 
earths. 

Pitchblende is possibly an amorphous uraninite containing a very 
little thoria and much water. Its specific gravity is often as low as 6.5, 
due probably to partial alteration. 

Uraninite crystallizes in the isometric system in octahedrons, and in 
combinations of O(in), 00 O(no), and 00 O 00 (100). Crystals are rare, 
however, the material usually occurring in crystalline masses and in 
botyroidal groups. 

The mineral is gray, brown or black and opaque. Its streak is 
brownish black, gray or olive green. Its luster is pitch-like or dull. Its 
fracture is uneven or conchoidal. It is brittle, its hardness is 5.5 and 
density 9-9.7. Like the other uranium minerals it is radioactive. 

Before the blowpipe uraninite is infusible. Some specimens color 
the flame green with copper. With borax it gives a yellow bead in the 
oxidizing flame, turning green in the reducing flame. All specimens give 
reactions for lead and many for sulphur and arsenic. The mineral is 
soluble in nitric and sulphuric acids, with slight evolutions of helium, 



COLUMBATES, TANTALATES AND URANATES 299 

the ease of solubility increasing with the increase in the proportion of 
rare earths present. 

Uraninite is distinguished from wolframite, samarskite, columbite and 
tantalite, by lack of cleavage, greater specific gravity, and differences in 
crystallization. From all but samarskite it is also distinguished by the 
reactions for uranium and, in the case of most specimens, by the reac- 
tion for lead. It is especially characterized by its pitch-black luster. 

Occurrence and Localities, — Uraninite occurs in pegmatites and in 
veins associated with silver, lead, copper and other ores. It is found in 
the ore veins in Saxony, Bohemia, and in pegmatites near Moss, Arendal 
and other points in Norway. 

In the United States it occurs in pegmatites at Middletown and 
Branchville, in Connecticut; at the Mitchell County mica mines, 
North Carolina; and at Barringer Hill, Llano County, Texas. It is 
also found in large quantity near Central City, Gilpin County, Colorado, 
where it is associated with gold, galena, tetrahedrite, chalcopyrite and 
other ore minerals. 

Production. — Uraninite has been mined in small quantity in Colo- 
rado, and at Barringer Hill, both as a source of uranium and as a 
source of radium. In Cornwall, England, and at Joachimsthal, 
Austria, it is mined as a source of radium. (See also p. 292.) 



CHAPTER XVI 

THE SILICATES 

The silicates are salts of various silicon acids, only a few of which 
are known uncombined with bases. The silicates include the commonest 
minerals and those that occur in largest quantity. They make up the 
greater portion of the earth's crust, forming most of the igneous rocks 
and a large portion of vein fillings. In number, the silicates exceed all 
other mineral compounds, but because of their stability they are of very 
little economic importance. A few are used as the sources of valuable 
substances, and their aggregates, the silicious rocks, are utilized as 
building stones, but, on the whole, they are of little commercial value. 
Since, however, they occur in good crystals and their material is trans- 
parent in thin sections so that it can easily be studied by optical methods, 
they are of great scientific importance. Much of the progress made in 
crystallography has been accomplished through the study of these com- 
pounds. 

Although the salts of the silicic acids are very numerous and most of 
them are very stable toward the ordinary reagents of the laboratory, 
the acids from which they are derived are only imperfectly known. 
The only one that has been prepared in the pure state is the compound 
H2Si03. This occurs as a gelatinous (colloidal) white substance which 
rapidly loses water upon drying and probably breaks up into a number 
of other compounds which are also acids, containing, however, a larger 
proportion of silicon in the molecule than that in the original compound. 
When the tetrafluoride, or the tetrachloride, of silicon is decomposed by 
water, the principal product is the acid referred to above, but in addition 
to this there is probably formed also the compound I^SiCU or Si(OH)4, 
which is the ortho acid. Some silicates are salts of these acids. Others 
are salts of the acids containing a larger proportion of silicon. In most 
cases, however, these acids may be regarded as belonging to a series in 
which the members are related to one another in the same manner as 
are normal sulphuric, common sulphuric and pyrosulphuric acids. Nor- 
mal sulphuric acid is HeSOe. By abstraction of 2H2O the compound 
H2SO4, or ordinary sulphuric acid, results. If from two molecules of 
H2SO4, one molecule of HoO is abstracted, H2S2O7, or pyrosulphuric 
acid, is left. In the same manner all of the silicic acids may be regarded 

300 



SILICATES 301 

as being derived from normal silicic acid Si(OH)4 or H4Si04 by the ab- 
straction of water, thus: 

Orthosilicic acid is RtSiC^, 
Metasilicic acid is H4Si04 — H2O or IfcSiOa, 
Diorthosilicic acid is 2H4Si()4— H2O or HeSi207, 
Dimetasilicic acid is 2H2Si03— H2O or H2S12O5, 
Trimetasilicic acid is 3H2Si03— H2O or HiS^Og. 

The compounds containing more than one silicon atom in the molecule 
are known as polysilicates. The salts of metasilicic acid are meta- 
silicates. 

Many attempts have been made to discover the chemical structure 
of the comparatively simple silicates and several proposals have been 
offered to explain the great differences often observed in the properties 
of silicates with the same empirical formula; but no explanation of these 
differences has thus far proved satisfactory. The silicates are so very 
stable under laboratory conditions, and, when they are decomposed, 
their decomposition products are so difficult to study, that it has been 
impossible to determine their molecular volumes or to understand their 
substitution products. We are thus driven to ascribe many of the 
anomalies in their composition to solid solutions, to absorption phenom- 
ena, and to the isomorphous mixing of compounds, some of which do 
not exist independently. 

There are many silicates, moreover, which cannot be assigned to any 
of the simple acids mentioned above, but which probably must be 
regarded as salts of very much more complex acids. Others are pos- 
sible salts of aluminosilicic acids in which aluminium functions in the 
acid portions. Thus, albite is usually regarded as a trisilicate, NaAlSiaOs, 
and anorthite as an orthosilicate, CaAl2(Si04)2. But the two substances 
are completely isomorphous, and for this reason it is thought that they 
must be salts of the same acid. If we assume an aluminosilicic acid of 
the formula HsA^Os, albite may be written (NaSi)AlSi20s, and anor- 
thite (CaAl)AlSi208. The two minerals thus become salts of the same 
acid and their complete isomorphism is explained. The relations that 
exist among many silicates might be better understood on the assump- 
tion that they are salts of complex silicic and of aluminosilicic acids 
than on the assumption that they are salts of simpler acids, as is now the 
case. But, since it has been impossible to isolate the acids and study 
them we are not certain as to their character. It is, therefore, believed 
best to represent most silicates as salts of the simplest acids possible, 
consistent with their empirical compositions as determined by analyses. 



302 DESCRIPTIVE MINERALOGY 

As in the case of salts of other acids there are silicates that contain 
hydrogen and oxygen in such relations to their other components that 
when heated they yield water. In some cases this water is driven off at 
a comparatively low temperature and the residue of the compound re- 
mains unchanged. A compound of this kind is usually called a hydrate 
or the compound is said to contain water of crystallization. In other 
cases a high temperature is necessary to drive off water, and the com- 
pound breaks up into simpler ones. In these instances the water is 
said to be combined. The compound is usually basic. 

In the descriptions of the silicates the order in which the minerals are 
discussed is that of increasing acidity, i.e., increasing proportion of the 
SiCfe group present in the molecule. This order, however, is not fol- 
lowed rigorously. The members of well defined groups of closely related 
minerals are discussed together even if their acidity varies widely. 
Nearly all the silicates are transparent or translucent and all are elec- 
trical insulators. 

THE ANHYDROUS ORTHOSILICATES 

NORMAL ORTHOSILICATES— R^SiC^ 
OLIVINE GROUP (R"flSi0 4 ). R" - Mg, Fe, Mn, Zn 

The members of the olivine group are normal silicates of the metals 
Mg, Fe, Mn and Zn. They constitute an isomorphous series crystalliz- 
ing in the holohedral division of the orthorhombic system (rhombic bi- 
pyramidal class). The most common member is the magnesium-iron 
compound (Mg-Fe)2Si04, olivine, or chrysot.le, from which the group 
gets its name. The members with the simplest composition are for- 
sterile (Mg2Si04), fayalitc (Fe2Si04) and tephroite (Mn2Si04). The 
others are isomorphous mixtures of these, with the exception of three 
rare minerals, of which one, monticeUUe, is a calcium magnesium silicate, 
another, titanolivine, contains Ti in place of a part of the Si, and the 
other, roepperite, contains some Zn2Si04. Most of them are formed 
by crystallization from molten magmas. 

Crystals of all the members of the group are prismatic and all have 
nearly the same habit. They are often flattened parallel to one of the 
pinacoids, oo P 06 (010) or 00 P 60 (100). The axial ratios of the com- 
moner members are as follows: 

Forsterite a : b : £=.4666 : 1 : .5868. The angle no A i7o=5o° 2' 

Olivine =.4658 : 1 : .5865. The angle no A 11*0=49 ° 57' 

Tephroite =.4600 : 1 : .5939. The angle no A 1^0=49° 24' 

Fayalite =4584 : 1 : .5793. The angle noAiio=49° *5' 



ANHYDROUS ORTHOSILICATES 



303 




m 



Crystals of olivine are usually combinations of some or all of the following 
forms: oo P 66 (ioo), oo P oo (oio), 
oP(ooi), ooP(no), ooP2(i2o), 

Poo (oil), 2Poo(o2l), Pob(lOl), 

P(iii) and 2P2(i2i) (Fig. 163). 
The crystals of fayalite are usually 
more tabular than those of olivine, 
but forsterite and tephroite crystals 
have nearly the same forms. The 
cleavage of all is distinct parallel 
to 00 P60 (010), less distinct parallel 
to 00 P 66 (100) in olivine, and par- 
allel to oP(ooi) in fayalite. 

The compositions of the pure Mg, 
Mn, and Fe molecules are: 





Fig. 163. — Olivine Crystals with 
00 P, no (m); 00 Poo, 010 (6); 
oP, 001 (c); 2P 00 , 021 (k); oo P2 , 
120 (*),P 00 , xoi (d) and P, in (e). 



MgO. 
MnO. 
FeO. 
Si0 2 . 



Mg2Si0 4 

- 57-x 



42.9 



Mn 2 Si0 4 
70.25 

29 -75 



Fe2Si04 



70.6 
29.4 



All natural crystals, however, contain some of all the metals indicated 
and, in addition, many specimens contain also a determinable quantity 
of CaO and traces of other elements. 



Forsterite, Olivine and Fayalite (Mg 2 Si0 4 - (Mg • Fe) 2 Si0 4 - Fe 2 Si0 4 ) 

The composition of olivine naturally depends upon the proportion 
of the forsterite and fayalite molecules present in it. When the propor- 
tion of FeO exceeds 24 per cent, the variety is known as hyalosiderite. 
A few typical analyses are quoted below: 



MgO 


FeO 


CaO 


AI2O3 


Si0 2 


Total 


Sp. Gr. 


I. 51.64 


5-oi 


1.08 


42 


42.30 . 


100.45 


3.261 


II. 50.27 


8.54 


• a • ■ 


• • • 


41.19 


100.00 




III. 48.12 


n. 18 


.12 


a • • 


40.39 


99.81 


3-294 


IV. 39.68 


22.54 


a • ■ • 


a a • 


37.17 


99 39 


• 



I. From masses enclosed in Vesuvian lava. 
II. Concretion in basalt near Sasbach, Kaisers tuhl. 

III. Grains from glacial debris, Jan Mayen, Greenland. 

IV. Grains from coarse-grained rock, near Montreal, Canada. 



a 


P 


7 


I. 6319 


1.6519 


I . 6698 


1 . 6674 


1 . 6862 


1 • 70S3 


1.8236 


1 . 8642 


1.8736 



304 DESCRIPTIVE MINERALOGY 

In addition, there are often also present small quantities of Ni, Mn, 
and Ti. 

Forsterite, olivine and fayalite are usually yellow or green in color 
and have a vitreous luster. Forsterite is sometimes white and olivine 
often brown. All three minerals become brown or black on exposure 
to the air. All are transparent or translucent. Their streak is colorless 
or yellow. The fracture of olivine is conchoidal. In the other two 
minerals it is uneven. Their hardness, density and refractive indices 
for yellow light are as follows: 

Hardness Sp. Gr. 

Forsterite 6-7 3 . 21-3 . 33 

Olivine 6.5-7 3- 2 7"3-37 

Fayalite 6.5 4.00-4.14 

Before the blowpipe most olivines and forsterites whiten but are in- 
fusible. Their fusion temperatures are between 1300 and 1450 , 
decreasing with increase in iron. Fayalite and varieties of olivine rich 
in iron fuse to a black magnetic globule. All three minerals are decom- 
posed by hydrochloric and sulphuric acids with the separation of gelat- 
inous silica; the iron-rich varieties are decomposed more easily than 
those poor in iron. 

The minerals are characterized by their color and solubility in 
acids. 

Both fayalite and olivine alter on exposure to the air, the former 
changing to an opaque mixture of Fe203 and Si02, or to the fibrous 
mineral anthophyllite ((Mg*Fe)Si03), and olivine to a mixture of 
iron oxides and fibrous or scaly gray or green serpentine (H4Mg3Si20Q). 
In other cases, under metamorphic conditions, the alteration is to a 
red lamellar mineral (iddingsite) which may be a form of serpentine, 
or to magnesite, or to the silicate, talc. Other kinds of alteration of 
this mineral have also been noted but those described are the most 
common. 

Syntheses. — The members of the olivine series have been produced 
by fusing together the proper constituents in the presence of magnesium 
and other chlorides. They are, moreover, present in many furnace 
slags where they have been made in the process of ore smelting. 

Occurrence. — Olivine occurs as an original constituent of basic igneous 
rocks and as a metamorphic product in dolomitic limestones. It is 
found also in the form of rounded grains in some meteoric irons. Fayalite 
occurs in acid igneous rocks, especially where affected by pneumatoiytic 



ANHYDROUS ORTHOSILICATES 305 

action, and forsterite in dolomitic rocks when they have been meta- 
morphosed by the action of igneous rocks. 

Localities. — Members of the olivine group occur in the basaltic lavas 
of many volcanoes — as those of the Sandwich Islands; in the limestone 
inclusions in the lava of Mt. Somma, near Naples; in various basic 
rocks in Vermont and New Hampshire and at Webster, N. C. At the 
latter place granular aggregates of almost pure olivine constitute great 
rock masses known as dunite, 

Fayalite is found in the rhyolites of Mexico, the Yellowstone Park 
and elsewhere, and in coarse granite at Rockport, Mass., and in the 
Mourne Mountains, Ireland. 

Forsterite occurs in limestone enclosures in the lava of Mt. Somma 
and at limestone contacts with igneous rocks at Bolton, Roxbury, and 
Littleton, Mass., and elsewhere. 

Uses and Production. — The only member of the group that is of any 
economic importance is a pale yellowish green transparent olivine, which 
is used as jewelry under the name of " peridot." Gem material is found 
at Fort Defiance and Rice, in Arizona, scattered loose in the soil. The 
little grains came from a basic volcanic rock. The amount produced in 
the United States during 191 2 was valued at about $8,100. 

Tephroite (Mn 2 Si0 4 ) 

Although tephroite is regarded as the manganese silicate it nearly 
always contains some of the forsterite molecule. 

Analyses of brown (I), and red (II), varieties from Sterling Hill 
gave: 

MnO FeO MgO CaO ZnO Loss Si0 2 Total 

I. 52.32 1.52 7.73 1.60 5.93 .28 30.55 99.93 

II.47.62 .23 14.03 .54 4.77 .35 31.73 99.27 

The mineral is gray, brown or rose-colored and transparent or 
translucent. Its streak is nearly colorless. It is rarely found in crys- 
tals. Its hardness is about 6 and its density 4.08. It is strongly 
pleochroic in reddish, brownish red and greenish blue tints. Its inter- 
mediate refractive index for yellow light = about 1.80. 

It is fusible with difficulty (fusing temperature =1200°), and is sol- 
uble in HC1 with separation of gelatinous silica. It is distinguishable 
from other like-appearing minerals by its difficult fusibility and its 
reaction with HC1. 

Syntheses. — Crystals of the mineral have been made by fusing to- 
gether Si02 and Mn02 in the proportion of 1 : 2, and by long-continued 



306 DESCRIPTIVE MINERALOGY 

heating of MnCk and Si02 in an atmosphere of moist hydrogen or carbon 
dioxide. 

Localities. — Tephroite occurs at Mine Hill and Sterling Hill, near 
Franklin, N. J., where it is associated with franklinite, zincite and 
troostite. It is found also at Pajsberg in Sweden with other man- 
ganese minerals and magnetite, and at Langban, in Wermland, 
Sweden. 

Uses. — The mineral is of little commercial value. It is separated 
with other manganese minerals from the zinc ore of Franklin, N. J., and 
is smelted with these in the production of spiegeleisen. 

WILLEMITE GROUP (R,"SiO*). R"=Zn, Mn. 

The willemite group comprises the two minerals -willemite (Zn2Si04) 
and troostite ((Zn-Mn^SiCU), of which the latter is rare. Willemite 
occurs in small quantity only, but troostite is an important source of 
zinc at the Franklin locality in New Jersey. Both minerals are found in 
crystals. 

Willemite and troostite crystallize in the rhombohedral hemihedral 
division of the hexagonal system (ditrigonal scalenohedral class), with 
the axial ratios 

Willemite a : c=i : 0.6698 
Troostite = 1 : 0.6698 

Willemite and Troostite (Zn 2 Si0 4 - (Zn Mn) 2 Si0 4 ) 

Willemite and troostite occur massive, in grains, and in simple crys- 
tals. 

The theoretical composition of willemite is Si02= 27.04 and ZnO 
= 72.96, but nearly all natural crystals contain traces of other elements. 
When a noticeable quantity of manganese is present, the compound 
is troostite. Several analyses are quoted below: 

SiO* 

Willemite from Stolberg, Germany 26 . 00 

Willemite from Greenland 27 . 86 

White troostite from Franklin, N. J.. . . 27. 20 
Dark red troostite from Franklin, N. J. . 27. 14 

The crystals of willemite exhibit the forms 00 R(ioTo), 00 P 2(1 120), 
oR(oooi), 111(3034) and — £R(oil2)(Fig.i64). Twins, with |P2 (3.3.6. 10) 
as the twinning planes, are rare. The crystals of troostite are even 
more simple, with ooP2(ii2o) and R(ioTi), usually the only forms 



ZnO 


MnO 


FeO 


Total 


72.91 


• • * • 


• 35 


100.16 


7I-SI 


• • • ■ 


37 


99-74 


65.82 


6-97 


23 


100.22 


64.38 


6.30 


1.24 


99.06 



ANHYDROUS ORTHOSILICATES 



307 





present, though — £R(oiT2), — |R(°33 2 ) and R 3 (2i3i) are also occa- 
sionally found. The angle 10I1 A 1101 = 63° 59'. The cleavage of 
willemite is distinct parallel to oP(oooi), and of troostite distinct 
parallel to ooP2(ii2o), and less perfect parallel to R(ioTi) and 
oR(oooi). 

Willemite is colorless, yellow, brown or blue. Troostite is green, 
yellow, brown or gray. The colored varieties of both minerals are 
translucent. Colorless willemite is transparent. Both minerals are 
vitreous in luster. Their hardness is between 
5 and 6 and density between 3.9 and 4.3. The 
refractive indices of willemite for yellow light 
are: u> =1.6931, €=1.7118. 

Both minerals glow when heated before the 
blowpipe and are fused with difficulty (about 
1484 °), and both gelatinize with HC1. Willem- 
ite gives the reaction for zinc with Co(N03)2 
on charcoal, and troostite gives, in addition, 
the reaction for manganese. 

Syntheses. — Willemite crystals have been 
made by the action of gaseous hydrofluo- 
silicic acid upon zinc, and by the action of 
silicon fluoride on zinc oxide at cherry-red temperature. 

Localities and Origin. — Willemite occurs in comparatively small quan- 
tity at only a few places, associated with other zinc minerals. In 
America it is found in colorless and black crvstals at the Merritt 
Mine near Socorro, New Mexico, associated with mimetite, wulfenite, 
cerussite, barite and quartz. 

Troostite occurs only at Sterling Hill and Franklin Furnace, N. J., 
but in such large quantity that it constitutes an important proportion 
of the zinc ore for which these localities are noted. It is associated with 
franklinite and zincite. Both willemite and troostite are results of 
magmatic processes. 



Fig. 164. — Willemite Crys- 
tal with 00 P2, 1 1 20 (a); 
R, ioii (r) and — JR, 
01 T2 (e). 



Phenacite (Be 2 SiC>4) 

The theoretical composition of the compound Be2Si(>4 is Si04 = 54.47, 
BeO= 45.53. Many of the analyses of phenacite show that it ap- 
proaches very closely to this. A specimen from Durango, Mexico, for 
example, is: 



SiO= 54.71, BeO=45-32, MgO+CaO=.i4. Total = 100.17. 




308 DESCRIPTIVE MINERALOGY 

Phenacite crystallizes in the rhombohedral tetartohedral division of 
the hexagonal system with a : c= i : 1.0661. It occurs in crystals pos- 
sessing many different types of habit and with many different combina- 
tions of forms. Perhaps ooP2(iiIo), ooP(ioTo), R(ioli), R 3 (2i3i) 
and — JR(oii2) are the most common (Fig. 165). Interpenetration 

twins are common at some localities. The 
cleavage is indistinct parallel to 00 P(io7o). 
The angle 10T1 Alioi = 63 24'. 

Phenacite is colorless or white or some 
light shade of yellow or pink. It is trans- 
parent or translucent and has a glassy luster. 
Its hardness is 7.5, and density about 3 and 
the refractive indices for yellow light are: 

Fig. i6 S .-Phenacite Crystal » =I - 6 542, c- 1.6700. It is infusible and 
with 00 P2, 1120 (a); « p, insoluble in acids. When heated with a 

_ -JP3 little soda before the blowpipe it affords a 

/ v 2 ' white enamel. The mineral is phosphores- 

cent and pyroelectric. 

Colorless phenacite resembles quartz and I.erderiie, and the yellow 
variety topaz. It is best distinguished from them by its crystalliza- 
tion. 

Syntheses. — Small crystals have been made by the fusion of a mix- 
ture of SiOa and beryllium oxide and borax, and by melting together 
beryllium nitrate, silica and ammonium nitrate. 

Localities. — Phenacite occurs at the Emerald Mines near Ekaterin- 
burg in the Urals; near Fremont, in the Vogesen; at Reckingen, in 
Switzerland; in Durango, Mexico; near Pike's Peak, at Topaz Butte, 
and at Mount Antero, in Colorado, and at Greenwood, in Maine. In 
all cases the mineral is probably a result of pneumatolysis. 

Uses, — The colorless phenacite is used to a slight extent as a gem. 

GARNET GROUP 
(R",R'",(Si04)s). R"=Ca, Mg, Fe, Mn. R"'=A1, Fe, Cr 

The garnet group comprises a large number of isomorphous com- 
pounds, some of which are very common. The members nearly all 
occur in distinct crystals that are combinations of isometric holohedrons 
(hexoctahedral class). Many different names have been given to the 
garnets and analyses show that they possess very different compositions. 
With the exception of a few rare varieties, they can all, however, be 
explained as consisting of one of the six molecules indicated below, or of 



ANHYDROUS ORT*HOSILICATES 



309 



mixtures of them. The six molecules and the names of the garnets 
corresponding to them, together with their densities, are: 

Ca3Al2(Si04)3 Grossularite or Hessonite Sp. gr. = 3.4-3.6 

Mg3Al 2 (Si04)3 Pyrope =3.7-3.8 

MnaAl2(Si04)3 S pes sar tile =4.1-4.3 

Fe3Al2(Si04)3 Almandite =4.1-4.3 

Ca3Fe2(SiC>4)3 Andradite or Mdanite =3.8-4.1 

Ca3Cr2(Si04)3 Uvarovite =3-4 

The following table contains the calculated percentage composition 
of the several pure garnet molecules and the records of analyses of some 
typical varieties of the mineral: 



It 

II* 

lib 

HI* 

III b 
IVa 

IV b 

Va 

v b 

Vo 
Via 
VI b 



SiO, Al 2 Os 

40.01 22.69 

42.01 17.76 

44.78 25.40 .. 

40.92 22.45 5 
36.30 20.75 •• 
36.34 12.63 4 
36.15 20.51 43 
37.61 22.70 33 

35-45 3i 

35 . 09 tr. 29 

26.36 22 

38. 23 

36.93 5- 6 8 1 



FcaOi CnO, FeO 



46 

• • 

57 

34 

83 

49 

15 
00 

• • 

96 



• • • • 



• • • • 



506 



8. 11 



MgO 


CaO 


• • • a a 


3730 


•13 


35 01 


29.82 


• • • • • 


17.85 


504 



.47 1.49 



2.49 



29.27 

21.84 



3.01 


1.44 


• a • • 


3306 


• 24 


3280 


1.25 


30.72 


• • • • 


29.27 


i-54 


3163 



MnO TiO, 



20 

• • 

46 

42.95 
44.20 

• • • • • 

12 



36 



tr. 



■ • • a a 

• • • • • 

• • a • • 
a • a a a 

21.56 

a a a a a 



Total 
100.00 
100.17 
100.00 
100.39 
100.00 

99.70 
100.00 
100.31 
100.00 
100.48 
101.89 
100.00 

9958 



la. Theoretical composition of the grossularite molecule. 

lb. Green and red grossularite from the limestone at Santa Clara, Cal. 

Ila- Theoretical composition of the pure pyrope molecule. 

lib. Pyrope from a peridotite in Elliot Co., Ky. Also, HjO ■» .10. 
Ilia. Theoretical composition of spessartite. 
IIIb. Spessartite from Amelia Court House, Va. 
IVa. Theoretical composition of almandite. 
IVb. Almandite from Salida, Colo. 

Va. Theoretical composition of andradite. 

Vb. Andradite from East Rock, New Haven, Conn. Also, HjO ** .35. 

V c . Schorlomite from Magnet Cove, Ark. 
Via- Theoretical composition of uvarovite. 
VIb- Uvarovite from Bissersk, Urals. 

The crystals of garnet are usually simple combinations of 00 O(no) 
(Fig. 166); 202(211) and often 3OK321) (Figs. 167 and 168), although 
all the other holohedrons are also occasionally met with. Their cleavage 
which is indistinct is parallel to 00 O(uo). 



310 DESCRIPTIVE MINERALOGY 

When examined in polarized light many garnets, especially those 
occurring in metamorphic rocks, are doubly refracting and, therefore, 
have not the molecular structure belonging to isometric crystals. This 



Fig. 166.— Garnet Crystal. (Natural size.) Form: °oO (no). 




Ficj. 167. Fie. 168. 

Fie. 167.— Garnet Crystals with «0, no (d) and 2O2, 211 (n). 
Fig. 168.— Garnet Crystal with d and « as in Fig. 16;. Also « O2, 210 (c) and 3 0|, 

131 w. 

phenomenon has been explained as due to several causes, the most rea- 
sonable explanation ascribing it to strains produced in the crystals upon 
cooling. 



ANHYDROUS ORTHOSILICATES 311 

The garnets vary in color according to their composition, the com- 
monest color being reddish brown. Their luster is vitreous, their 
streak white, hardness 6-7.5, an d density 3.4-4.3. They are transparent 
or translucent. Most varieties are easily fusible to a light brown or 
black glass, which in the case of the varieties rich in iron is magnetic. 
Uvarovite, however, is almost infusible. Some garnets are unattacked 
by acids; others are partially decomposed. 

Garnets, when in crystals, are easily distinguished from other sim- 
ilarly crystallizing substances by their color and hardjiess. Massive 
garnet may resemble resuzianilc t sphene, zircon or tourmaline. It is 
distinguished from zircon by its easier fusibility and from vesuvianite 
by its more difficult fusibility; from tourmaline by its higher specific 
gravity, and from sphene by the reaction from titanium. 

Under the influence of the air and moisture garnets may be partially 
or entirely changed to epidote, muscovite, chlorite, serpentine, and oc- 
casionally to other substances. 

Grossularite, Essonite, Hessonite, or Cinnamon Garnet occurs 
principally in crystalline schists and in metamorphosed limestones, 
where it is associated with other calcium silicates. It is found also 
in quartz veins. The mineral is white, bright yellow, cinnamon-brown 
or some pale shade of green or red. The lighter-colored varieties are 
transparent or nearly so. Those that are colored are used as gems. 
Much of the hyacinth of the jewelers is a red grossularite (see p. 317). 
Its hardness is about 7 and its density 3.4-3.6. It is fairly easily 
fusible before the blowpipe. The refractive index of colorless vari- 
eties for yellow light is, »= 1.7438. 

Good crystals of grossularite occur at Phippsburg, Raymond and 
Rumford, in Maine, and at many other places both in this country and 
abroad. Bright yellow varieties are reported from Canyon City, Colo. 

Pyrope is deep red, sometimes nearly black. Its hardness is a little 
greater than 7 and its density 3.7. Its refractive index for yellow light 
is between 1.74 12 and 1.7504. The pure magnesium garnet is unknown. 
All pyropes contain admixtures of iron and calcium molecules. Many 
pyropes are transparent. Those with a dark red color are used as gems. 
They occur principally in basic igneous rocks. 

The principal occurrence of the gem variety in this country is in 
Utah, near the Arizona line, .about 100 miles west of Ganado, Ariz., 
where it is found lying loose in wind-blown sand. 

Rhodolite is a pale rose-red or purple variety from Macon Co., N. C. 
It consists of two parts pyrope and one of almandite, 



312 DESCRIPTIVE MINERALOGY 

Spessartite is hyacinth or brownish red, with occasionally a tinge 
of violet. The purest varieties are yellow, but since there is nearly 
always an admixture of one of the iron molecules, the more usual color 
is reddish brown. The mineral is usually transparent. Its hardness is 
7 or a little greater, and its density 3.77-4.27. Its refractive index for 
yellow light is 1.8105. In the blowpipe flame it fuses fairly easily to a 
black, nonmagnetic mass, and with borax gives an amethyst bead. It 
is found in acid igneous rocks and in various schists. 

Its best known occurrences in the United States are in granite, at 
Haddam, Conn., in pegmatite, at Amelia Court House, Va., and in 
the lithophyse of rhyolites, near Nathrop, in Colorado. 

Almandite is deep red, brownish red or black. It is one of the com- 
monest of all garnets. It furnishes nearly all the material manufactured 
into abrasives. Transparent varieties are also used as gems. The min- 
eral has a hardness of 7 and over. Its density is 4.1-4.3, and its refrac- 
tive index, n, for yellow light, is about 1.8100. It is slightly decom- 
posed by HC1. Before the blowpipe it fuses to a dark gray or black 
magnetic mass. It is found in granites and andesites, and also in various 
gneisses and schists and in ore veins. 

Its best known occurrences in North America are at Yonkers and 
at various points in the Adirondacks, N. Y., at Avondale, Pa., and on the 
Stickeen River, in Alaska. 

Andradite, or melanite, is black, brown, brownish red, green, brown- 
ish yellow or topaz-yellow. The purest varieties are topaz-yellow or 
light green and transparent. The former constitute the gem iopazolite 
and the latter, demantoid. The black variety, melanite, nearly always 
contains titanium. It occurs in alkaline igneous rocks, in serpentine, 
in crystalline schists and in iron ores. The most titaniferous varieties 
are known as schorlomite. The hardness of andradite is about 7 and its 
density between 3.3 and 4.1. n for yellow light = 1.8566. It is fusible 
before the blowpipe to a black magnetic mass. 

The mineral is very widely spread. It occurs at Franklin, N. J., in 
metamorphosed limestone; near Franconia, N. H., in quartz veins, and 
at many other places. A black titaniferous variety occurs in a meta- 
morphosed limestone in southwestern California and near Magnet Cove, 
in Arkansas. The variety found at Magnet Cove is schorlomite. It is a 
black glassy mineral associated with brookite (TiCfe), nepheline (p. 314), 
and thomsonite (p. 455). 

Common garnet is a mixture of the grossularite, almandite and 



ANHYDROUS ORTHOSILICATES 313 

andradite molecules. It occurs in many metamorphosed igneous rocks 
and in some slates. 

Uvarovite is emerald-green. It is rare, occurring only with chromite 
in serpentine at Bissersk and Kyschtim in the Urals and in the chromite 
mines at Texas, Penn., and New Idria, Cal. Its hardness is about 7 
and density 3.42. Its refractive index for yellow light is 1.8384. It is 
infusible before the blowpipe but dissolves in borax, producing a green 
bead. 

Syntheses. — Garnet crystals have been produced by fusing 9 parts of 
nepheline and 1 part of augite (p. 374). The fusion results in a 
crystalline mass of nepheline, in which spinel and melanite crystals Are 
embedded. 

Occurrence. — The members of the garnet group are widely spread in 
nature. They occur in schists, slates and other regionally metamor- 
phosed rocks, in granite, rhyolite and other igneous rocks, and as con- 
tact products in limestones. They are found also in quartz veins, in 
pegmatite, and associated with other silicates in ore veins. In some 
instances they separated from a cooling magma, in others they are the 
products of pneumatolitic process, and in others they are the results of 
contact and dynamic metamorphism. 

Uses and Production. — The varieties that are transparent are used 
as gems. Other varieties are crushed and employed as abrasives. The 
value of the gem material produced in the United States in 191 2 was 
$860. The production for abrasive purposes was 4,182 short tons, val- 
ued at $137,800. All of this was produced in the mountain regions of 
New York, New Hampshire and North Carolina. The rock is crushed 
and the garnet separated by hand picking, screening, or by jigging. 
The crushed material is used largely in the manufacture of garnet paper. 

NEPHELINE GROUP 

The nepheline group of minerals includes three closely related com- 
pounds, of which nepheline is the most common. They are all alumino- 
silicates of the alkalies. Nepheline appears to be a solution of Si02, 
or of albite, in isomorphous mixtures of the orthosilicates, NaAlSi04 
and KAlSi04 in the proportion of 8 molecules of the silicates to one of 
Si02, thus: 

8(Na- K)AlSi0 4 +Si0 2 = (Na- K) ((Na- K)AlSi0 3 ) 2 Al 6 (Si0 4 )7 

The other two members of the group are eucryptite (LiAlSi0 4 ) and 
kaliophilite (KAlSi0 4 ). 



314 



DESCRIPTIVE MINERALOGY 



The members of the group crystallize in the hexagonal system and 
are apparently holohedral, but nepheline is hemihedral and hemi- 
morphic (hexagonal pyramidal class). At temperatures above 1,248° 
the nepheline molecule crystallizes also in the triclinic system as car- 
negieUc (see p. 418). 



Nepheline ((Na-K)8Al 8 Si©034) 

Although approximately a potash-soda silicate, nearly all specimens 
of nepheline contain more or less CaO and nearly all contain small 
quantities of water. All contain an excess of SiC>2. To avoid the 
necessity of assuming the existence of this SiC>2 in solution with 
(Na* K)AlSiC>4, it has been suggested that the variable composition of 
the mineral may be explained by regarding it as a solid solution of 
NaAlSiaOs and CaAkSi^s (best known in their triclinic forms as 
albite and anorthite) in an isomorphous mixture of the two molecules, 
NaAISiCU and KAlSiO*. The average of five analyses of crystals from 
Monte Somma, Italy, is shown in I, and the composition of a mass of 
the mineral from Litchfield, Maine, in II. 



Si0 2 AI2O3 

I.44.08 33.28 

II. 43 .74 34 • 48 



CaO MgO Na 2 K 2 H 2 

1.57 .19 16.00 4.76 .15 
tr tr 16.62 4.55 .86 



Total 
100.03 
100.25 



m 



m 



m 



When found in crystals, the mineral is apparently holohedral in form 
with an axial ratio 1 : .8389. The crystals are nearly always short 

columnar in habit and usually consist of very 
/< ^ c ^ >\ simple combinations. The most prominent 
NX p A^-il forms are 00 P(ioTo), ooP2(ii2o), oP(oooi), 

2P(202l), P(ioll), £P(l0l2) and 2P2(lI2l) 

(Fig. 169). Their cleavage is imperfect parallel 
to 00 P( 10T0) and oP (0001 ) . 

Nepheline is glassy, white or gray and trans- 
parent, when occurring as implanted crystals. 

xt 1 i- ^ The translucent varietv with a glassy luster 
Fig. 169 —Nepheline Crys- . . - * f 

tal with oP 0001 U)' ^ at occurs m rocks is known as eleohte. This 
00 p, 10T0 (w); P, 10T1 variety may be gray, pink, brown, yellowish or 
(p) and 00 P2, 1 1 20 (a), greenish. The streak is always white. The 

fracture of both forms is conchoidal or uneven; 
hardness, 5-6 and density, 2.6. For yellow light, co= 1.5424, 6=1.5375. 



ANHYDROUS ORTHOSILICATES 315 

Before the blowpipe nepheline melts to a white or colorless blebby 
glass. At 1,248° it passes over into carnegieite which melts at 1,526°. 
It dissolves in hydrochloric acid. with the production of gelatinous 
silica. Its powder before and after roasting reacts alkaline. 

The mineral is distinguished from other silicates by its crystalliza- 
tion, gelatinization with acids, and hardness. The massive varieties 
are often distinguishable by their greasy luster. 

Nepheline alters to various hydrated compounds, especially to the 
zeolites (p. 445), and to gibbsite, muscovite, cancrinite and sodalite. 

Syntheses. — Nepheline has been prepared by fusing together AI2O3, 
Si02 and Na2CC>3, and by the treatment of muscovite by potassium 
hydroxide. 

Occurrence. — The mineral occurs principally as an original constit- 
uent of many igneous rocks, both plutonic and volcanic, and also as 
crystals on walls of cavities in them. 

Localities. — Crystals occur near Eberbach, in Baden; in the inclu- 
sions within volcanic rocks at Lake Laach, in Rhenish Prussia; in the 
older lavas of Monte Somma, Naples, Italy; at Capo de Bove, near 
Rome; in southern Norway; and at various other points in southern 
Europe. Massive forms are found in coarse-jjrained rocks near Litch- 
field, Maine; Red Hill, N. H.; Magnet Cove, Ark. ; in the Crazy Mts., 
Mont., and at other places. 



Cancrinite (He^CaMNaCOa^AlsCSiO^) 

Cancrinite is extremely complex in composition. It is nearly allied 
to nepheline but contains a notable quantity of CO2. It corresponds 
approximately to an hydrated admixture of Na2CC>3 and 3NaAlSi04, 
in which some of the Na is replaced by K and Ca. Specimens from 
Barkevik (I) in Norway, and from Litchfield (II), in Maine, yield the 
following analyses: 

Si0 2 AI2O3 Fe 2 3 CaO Na 2 K 2 C0 2 H 2 Total 
I. 37.01 26.42 .... 7.19 18.36 .... 7.27 3.12 99.37 
II. 36.29 30.12 tr. 4.27 19.56 .18 6.96 2.98 100.36 

Cancrinite is hexagonal (dihexagonal bipyramidal class). 

Crystals are rare, and those that do exist are very simple, prismatic 
forms bounded by ooP(ioTo), ooP2(ii2~o), oP(oooi) and P(ioTi). 
Their axial ratio is 1 : .4410. 



316 DESCRIPTIVE MINERALOGY 

The mineral is usually found without crystal planes. It is colorless, 
white or some light shade, such as rose, bluish gray or yellow. Its 
streak is white, its luster glassy, greasy or pearly and it is translucent. 
Its cleavage is perfect parallel to ooP(ioTo) and less perfect parallel 
to oo P2 11 20). Its break is uneven; hardness 5 and density 2.45. 
For red light: w= 1.5244, €=1.4955. 

Before the blowpipe the mineral loses its color, swells and fuses to a 
colorless blebby glass. In the closed glass tube it loses CO2 and water, 
and becomes opaque. After roasting it is easily attacked by weak 
acids with effervescence and the production of gelatinous silica. When 
boiled with water Na2CC>3 is extracted in sufficient quantity to give an 
alkaline reaction. 

Cancrinite is easily distinguished by its effervescence with acids and 
the production of gelatinous silica. 

Synthesis. — Small colorless, hexagonal crystals with a composition 
corresponding to that of cancrinite, have been made by treating mus- 
covite with a solution of NaOH and NaoCCb at 500 . 

Occurrence. — The mineral occurs principally as an associate of neph- 
eline in certain coarse-grained igneous rocks. In some cases it appears 
to be an original rock constituent and in others an alteration product of 
nepheline. It sometimes alters to natrolite (see p. 454), forming pseu- 
domorphs. 

Localities. — Cancrinite is found in rocks at Ditro, Hungary; at 
Barkevik and other localities in southern Norway, where it occurs in 
pegmatite dikes; in the parish of Kuolajarvi, in Finland, and in nepheline 
syenite at Litchfield in Maine. 

ZIRCON GROUP 

The orthosilicates of zirconium, zircon, and of thorium, thorite, con- 
stitute a group, the members of which possess forms that are almost 
identical with those of rutile, cassiterite and xenotime. Indeed, parallel 
growths of zircon and xenotime are not uncommon. Formerly zircon 
was grouped with the two oxides. 

Zircon and thorite are tetragonal (ditetragonal bipyramidal class), 
with approximately the same axial ratios and the same pyramidal angles. 
The two minerals are completely isomorphous. 

Zircon ZrSi04 a : £=.6301 in A 1^1 = 56° 37', 
Thorite ThSi0 4 =.6402 =56° 40'. 

Zircon is fairly common. Thorite is rare. 



ANHYDROUS ORTHOSILICATES 



317 



Zircon (ZrSi0 4 ) 

Zircon, like rutile, is a fairly common compound of a comparatively 
rare metal. It is practically the only ore of the metal zirconium. It is 
found mainly in crystals and as gravel. 

Although some specimens of zircon contain a large number of ele- 
ments, others consist only of zirconium, silicon and oxygen in propor- 
tions that correspond to the formula ZrSiCU, which demands 67.2 per 
cent ZrO and 32.8 per cent Si02. 

Its axial ratio is a : f=i : .6391. Its crystals are usually simple 
combinations of 00 P(no) and P(m), with the addition of 00 P 00 (100) 





m 



m 






Fig. 170. 



Fig. 171. 



Fig. 170. — Zircon Crystals with 00 P, no («); 00 Poo, 100 (a); 3P, 331 (*), 

P, in (p) and 3P3, 311 (x). 
Fig. 171. — Zircon Twinned about P 00 (101). *=2P (221). 



and often 3P3(3ii) (Fig. 170). Elbow twins, like those of rutile and 
cassiterite, are known (Fig. 171). 

The cleavage of zircon is very indistinct. Its fracture is conchoidal. 
Its hardness is 7.5 and density about 4.7. The mineral varies in tint 
from colorless, through yellowish brown to reddish brown. Its streak 
is uncolored and luster adamantine. Most varieties are opaque, but 
transparent varieties are not uncommon. The orange, brown and red- 
dish transparent kinds constitute the gem known as hyacinth. The 
refractive indices for yellow light are: «= 1.9302, c= 1.9832. 

Zircon is infusible, though colored varieties often lose their color 
when strongly heated. In the borax and other beads the mineral gives 
no preceptible reactions. In fine powder it is decomposed by concen- 
trated sulphuric acid. When fused with sodium carbonate on platinum 
it is likewise decomposed, and the solution formed by dissolving the 
fused mixture in dilute hydrochloric acid turns turmeric paper orange. 
This is a characteristic test for the zirconium salts. 



318 DESCRIPTIVE MINERALOGY 

The mineral is easily recognized by its hardness, its resistance toward 
reagents and its crystallization. 

Syntheses. — Small crystals of zircon are obtained by heating for sev- 
eral hours in a steam-tight platinum crucible a mixture of gelatinous 
silica and gelatinous zirconium hydroxide. Crystals have also been 
made by heating for a month a mixture of ZnCfe and S1O2 with 6 times 
their weight of lithium bimolybdate. 

Occurrence and Origin. — Zircon is widely spread in tiny crystals as a 
primary constituent in many rocks, and in large crystals in a few, notably 
in limestone and a granite-like rock known as nepheline syenite. In 
limestone it is a product of contact action. It occurs also in sands, 
more particularly in those of gold regions, and abundantly in a sand- 
stone near Ashland, Va. 

Localities. — The principal occurrences of the mineral are Ceylon, the 
home of the gem hyacinth; the gold sands of Australia; Arendal, 
Hakedal and other places in Norway; Litchfield and other points in 
Maine; Diana, in Lewis Co., and a large number of other places in New 
York; at Reading, Penn.; Henderson and other Counties, in North 
Carolina and Templeton, Ottawa Co., Quebec. 

Uses. — Zircon is the principal source of the zirconium oxide employed 
in the manufacture of gauze used in incandescent gas lights and in the 
manufacture of cylinders for use in procuring a light from the oxyhydro- 
gen jet. The mineral has been mined for these purposes in Henderson 
Co., North Carolina. 

Transparent orange-colored zircons are sometimes used as gems 
since they possess a high index of refraction and consequently have 
a great deal of " fire." These are the true .hyacinth. The mineral 
often called by this name among the jewelers is a yellowish brown 
garnet. 

Production. — A small quantity of zircon is usually obtained from 
Henderson Co., N. C, but it rarely amounts to more than a few hundred 
pounds. The mineral occurs in a pegmatite and the soil overlying its 
outcrop. It is obtained by crushing the rock and hand picking. Usually 
there is a little also separated from the sands in North Carolina and 
South Carolina that are washed for monazite. A pegmatite dike, rich 
in zircon, is also being prospected in the Wichita Mountains, Okla., but 
no mining has yet been attempted. 




ANHYDROUS ORTHOSILICATES 319 

Thorite (ThSi0 4 ) 

Thorite occurs in simple crystals bounded by oo P(no) and P(m) 
(Fig. 172), and in masses. The mineral is always 
more or less hydrated, but this is believed to be 
due to partial weathering. It is black or orange- 
yellow (orangeite), has a hardness of 5 and a specific 
gravity of 4.5-5 for black varieties and 5.2-5.4 for 
orange varieties. Its streak is brown or light orange. 
Hydrated specimens are soluble in hydrochloric acid 
with the production of gelatinous silica. The min- Fig. 172.— Thorite 
eral occurs as a constituent of the igneous rock, Crystal with *> p, 
augite-syenite, at several points in the neighborhood IIQ '*' an ' 
of the Langesundf jord, Norway, 

BASIC ORTHOSILICATES 

ANDALUSITE GROUP 

Three compounds with the empirical formula AkSiOs exist as min- 
erals, kyanite, or disthene, andalusite and sUlimanile. The first named is 
less stable with reference to chemical agents than the other two, but at 
high temperatures both kyanite and andalusite are transformed into 
sillimanite. Kyanite is regarded as a metasilicate (A10)2SiC>3. The 
other two are thought to be orthosilicates (Al(AlO)SiC>4). The latter 
are orthorhombic and both possess nearly equal prismatic angles. 
They differ markedly, however, in their optical and other physical 
properties and, therefore, are different substances. Kyanite is triclinic. 
For this reason and because of its different composition it is not re- 
garded as a member of the andalusite group. A fourth mineral, topaz, 
differs from andalusite in containing fluorine. Often this element is 
present in sufficient quantity to replace all of the oxygen in the radical 
(AlO). In other specimens the place of some of the fluorine is taken 
by hydroxyl (OH). The general formula that represents these varia- 
tions is Al(Al(F-OH)2)Si04. The mineral crystallizes in forms that are 
very like those of andalusite, and if corresponding pyramids are selected 
as groundforms their axial ratios are nearly alike. Unfortunately, 
however, different pyramids have been accepted as groundforms, and 
therefore the similarity of the crystallization of the two minerals has 
been somewhat obscured. Danburite, another mineral that crystallizes 
in the orthorhombic system with, a habit like that of topaz is often also 
placed in this group, although it is a borosilicate, thus CaB2(Si04)2. 



320 



DESCRIPTIVE MINERALOGY 



If 4P2(24i) be taken as the groundform of andalusite, 3P(33i) as 
that of topaz and 3P(33i) as that of danburite, the corresponding axial 
ratios would be: 

Andalusite a : b : c— .5069 : 1 : 1 .4246 
Topaz =.5281 : 1 : 1.4313 

Danburite = . 5445 : 1 : 1 . 4402 

These, however, are not the accepted ratios, since other and more prom- 
inent pyramids have been selected as the groundforms. 

Andalusite and Sillimanite (Al(A10)Si0 4 ) 

Andalusite and sillimanite have the same empirical chemical compo- 
sition and crystallize with the same symmetry, which is orthorhombic 
holohedral (rhombic bipyramidal class), but they have different physical 
properties and different crystal habits, and hence are regarded as dif- 
ferent minerals. The theoretical composition of both is: 

Si02= 37.02; Al203 = 62.98. Total= 100.00. 

Nearly all specimens when analyzed show the presence of small 
quantities of Fe, Mg, and Ca, but otherwise they correspond very closely 
to the theoretical composition. 

Both minerals are characteristic of metamorphosed rocks, but 
andalusite occurs principally in those that have been metamorphosed by 

contact with igneous intru- 
sives, while sillimanite is 
especially characteristic of 
crystalline schists and, in gen- 
eral, of rocks that were dy- 
namically metamorphosed. It 
also occurs with olivine as in- 
clusions in basalt lavas. Silli- 





m 



m 



_V 



Fig. 173.— Andalusite Crystals with <» p, no manite is more stable at high 

j w »;^ l( * P»£ii_(j); oops 100 temperatures than andalusite. 

/ x r>- OI ° ,\ t* 2> "/^v J" t>-' When in contact rocks it is 

120 (»); Poo, 101 (r); P, 111 (p) and 2P2, 

121 (£). found nearer the intrusive 

than andalusite. 
Andalusite. — The accepted axial ratio of andalusite is .9861 : 1 : .7024. 
Its crystals are columnar in habit and are usually simple combinations 

of 00 P GO (lOo), 00 P 06 (OIO), OP(OOI), 00 P(lio), 00 P2(2I0), 00 P2(l20) 

P« (101), Poo (on) with sometimes P (in) and 2P2(i2i) (Fig. 173). 
The angle no A 110=89° I2 '- 



ANHYDROUS ORTHOSILICATES 321 

The mineral, when fresh, is greenish or reddish and transparent. 
Usually, however, it is more or less altered, and is opaque, or, at most, 
translucent, and gray, pink of violet. Its cleavage is good parallel to 
oo P(no) and its fracture uneven. Its hardness is 7 or a little less and 
its density 3.1-3.2. In some specimens pleochroism is marked, their 
colors being olive-green for the ray vibrating parallel to a, oil-green for 
that vibrating parallel to b and dark red for that vibrating parallel to c. 
For yellow light the indices of refraction are: 0=1.6326, 18=1.6390, 
7=1.6440. 

Before the blowpipe the mineral gradually changes to sillimanite and 
is infusible. When moistened with cobalt nitrate and roasted it becomes 
blue. It is insoluble in acids. 

The mineral is distinguished by its nearly square cross-section, its 
hardness, its infusibility, and the reaction for Al, and by its manner of 
occurrence in schists and metamorphosed slates. 

Some specimens contain as inclusions large quantities of a dark 
gray or black material, which may be carbonaceous, arranged in 
such a way as to give a cross-like figure in cross-sections of crystals. 
Because of the shape of the figure exhibited by these crystals, this 
variety was early called chiastolite, and was valued as a sacred 
charm. 1 

Andalusite alters readily to kaolin (p. 404), muscovite (p. 355), and 
sillimanite. It has not been produced artificially. 

Occurrence. — Andalusite is found principally in clay slates and schists 
that have been metamorphosed by contact with igneous masses, and 
to a less extent in gneisses. 

Localities. — Its principal occurrences are in Andalusia, Spain; at 
Braunsdorf, Saxony; at Gefrees, in the Fichtelgebirge; in Minas 
Geraes, Brazil, and in the United States at Standish, Maine; Westford, 
Mass., and Litchfield, Conn. Chiastolite occurs at Lancaster and 
Sterling, Mass. 

Use. — The only use to which andalusite has been put is as a semi- 
precious stone, and for this purpose only the chiastolite variety is of any 
value. 

Sillimanite, or fibrolite, occurs principally in acicular or fibrous 
aggregates, on the individuals of which only the prismatic forms 
ooP(no) and 00 P|(23o) and the macropinacoid 00 Poo (100) can be 
detected. End faces are not sufficiently developed to warrant the 
determination of an axial ratio. The relative values of the a and b 
axes are .687 : 1. The angle no A 110=69°. 

While most of the fibers correspond in composition very closely to the 



322 DESCRIPTIVE MINERALOGY 

theoretical value demanded by the formula Al(A10)Si04, many contain 
small quantities of Fe^, MgO and H2O. 

The mineral is yellowish gray, greenish gray, olive-green or brownish. 
It has a glassy or greasy luster and when pure is transparent. Most 
specimens, however, are translucent, and many of the colored varieties 
show a pleochroism in brown or reddish tints. Its cleavage is perfect 
parallel to 00P60 (100). Its needles have an uneven fracture trans- 
versely to their long directions. Their streak is colorless, hardness 
6-7 and density 3.24. The indices of refraction for the lighter colored 
varieties are: a= 1.6603, £=1.6612, 7=1.6818 for yellow light. 

Sillimanite reacts similarly to andalusite toward reagents and before 
the blowpipe. It is distinguished from other minerals by its habit and 
manner of occurrence. 

This mineral is much more resistant to weathering than is andalusite. 
It is, however, occasionally found altered to kaolin. On the other hand, 
it is known also in pseudomorphs after corundum. 

Synthesis. — It has been produced by cooling fused silicate solutions 
rich in aluminium. 

Occurrence. — Sillimanite is very widely spread in schistose rocks, 
especially those that have been formed from sediments. It is essentially 
a product of dynamic metamorphism, but is formed also bv contact 
metamorphism, in which case it is found near the intrusive, where the 
temperature was high. 

Localities. — Its principal occurrences in North America are in quartz 
veins cutting gneisses at Chester, Conn., at many points in Delaware 
Co., Penn., and at the Culsagee Mine, Macon Co., N. C. At the lAtter 
place and at Media in Penn., a fibrous variety occurs in such large 

masses as to constitute a schist — known as fibrolite schist. • r -\ 

{ • 

Topaz (Al(Al(F-0H) 2 )Si0 4 ) 

Topaz is a common constituent of many ore veins and is often present 
on the walls of cracks and cavities in volcanic rocks. It occurs massive 
and also in distinct and handsome crystals. 

The mineral has a varying composition, which is explained in part 
by the fact that it is a mixture of the two- molecules Al(AlF2)Si04 and 
Al(Al(OH)2)SiOi. The theoretical composition of the fluorine molecule 
is: Si02=32.6, Al203 = 55.4; F= 20.7= 108.7; deduct (0=2F)8.7 
s= 100.00. A specimen from Florissant, Colo., gave: 

Si0 2 =33- I 5; Al 2 O a = 57.01; F =16.04 =106.20- 6.75(0 «=F) = 99.45. 



ANHYDROUS ORTHOSILICATES 



323 



Crystals of topaz appear to be orthorhombic (rhombic bipyramidal 
class), but the fact that they are pyroelectric and that they frequently 
exhibit optical phenomena that are not in accord with the symmetry of 
orthorhombic holohedrons suggests that they may possess a lower grade 
of symmetry. On the assumption that the mineral crystallizes with the 
symmetry of orthorhombic holohedronf the axial ratio of fluorine varie- 
ties is .5281 :.i : -477r. 1 With the increasing presence of OH, however, 
the relative length of a increases and that of c diminishes. The angle 
noAiTo=S5° 43'- 

The crystals are usually prismatic in habit with w>P(no) and 
00 PJ(i2o) predominating. They are notable for the number of forms 



f&>\ 



Fie. 174- 
FlG.174.— Topaz Crystals with »P,iio{m); °° 1 
4P » , 041 00 and »P« 
Fic. 175.— Topaz Crystal with m, I, n and y as 

*P«,043(A-)and2P« 



Fig. i 7S . 

r,i»(0; P.iiiMi iP»niM 

>.o (6). 

1 Fig. 174. Also 2p«,o 2 i (J); 

2 OI (d). 



that have been observed on them, especially in the prismatic zone and 
among the brachypyramids. The number of the latter that have 
already been identified is about 45. 

The three types of crystals that are most common are shown in 
Figs. 174, 175 and 176. Their most prominent forms are oop(no), 
■oPa(iao), P»(on), P(ni), |P(23 3 ). 4P*(o4i), « P3d3°) and 
oP(ooi). Often planes are absent from one end of the vertical axis, 
but since the etch figures on the prismatic planes do not indicate hemi- 
morphism, the absence of the lacking planes is explained as being due to 
unequal growth. The planes of the prismatic zone are usually striated. 

The mineral is colorless, honey yellow, yellowish red, rose and rarely 
bluish. When exposed to the sunlight the colored varieties fade, and 

inly accepted axial ratio is a : b : t= .5^85 :i : .0539, the form 



Then 



aP(aai) being taken as the groundform. 



324 



DESCRIPTIVE MINERALOGY 





m 



when intensely heated some honey-yellow crystals turn rose-red. Its 
cleavage is perfect parallel to oP(ooi) and imperfect parallel to P 06 (on) 
and P * (101). The hardness of the mineral is 8 and its density 3. 5-3. 6. 
Its refractive indices for yellow light are: a= 1.6072, 0= 1.6104, 7= 1.6 176 
for a variety containing very little OH, and a =1.6294, 0=1.6308, 

7=1.6375 for a variety rich in 
hydroxyl. The indices of refraction 
being high, the mineral when cut 
exhibits much brilliancy — a feature 
which, together with its hardness, 
gives it much of its value as a 
gem. 

Topaz is infusible before the 

blowpipe and is insoluble in acids. 

Fig. 176.— Topaz Crystal with m, I, y, At a high temperature it loses its 

/", d, O and u as in Figs. 174 and 175. fluorine as silicon and aluminium 

Also JP, 223 (0; oP, 001 (c) and fl uor ides. The mineral also ex- 

9 4 hibits pyroelectrical properties, but 

these are apparently distributed without regularity in different 

crystals. Many crystals contain inclusions of fluids containing bubbles, 

and sometimes of two immiscible fluids the nature of which has not vet 

been determined. It has been thought that the principal fluid present 

is liquid carbon-dioxide or some hydrocarbon. 

The mineral is distinguished from yellow quartz by its crystalliza- 
tion, its greater hardness and its easy cleavage. 

Topaz is frequently found coated with a micaceous alteration product 
which may be steatite (p. 401), muscovite (p. 355) 01 kaolin (p. 404). 

Synthesis. — Crystals have been made by the action of hydrofluosilicic 
acid (I^SiFe) upon a mixture of silica and alumina in the presence of 
water at a temperature of about 500 °. 

Occurrence. — The mineral occurs principally in pegmatites, espe- 
cially those containing cassiterite, in gneisses, and in acid volcanic rocks. 
In all cases it is probably the result of the escape of fluorine-bearing 
gases from cooling igneous magmas. 

Localities. — Topaz is found in handsome crystals at Schneckenstein 
in Saxony, in a breccia made up of fragments of a tourmaline*quartz 
rock cemented by topaz. It occurs also in the pegmatites of the tin 
mines in Ehrenfriedersdorf, Marienberg and other places in Saxony, 
Bohemia, England, etc.; on the walls of cavities in a coarse granite in 
Jekaterinburg and the Ilmengebirge, Russia; in veins of kaolin cutting 
a talc schist in Minas Geraes in Brazil; and in the cassiterite-bearing 



ANHYDROUS ORTHOSILICATES 325 

sands at San Luis Potosi, Durango and other points in Mexico. In the 
United States it occurs on the walls of cavities in acid volcanic rocks, at 
Nathrop, Colo., in the Thomas Range, Utah, and other places. It occurs 
also in veins with muscovite, fluorite, diaspore and other minerals at 
Stoneham, Maine, and Trumbull, Conn. 

Uses and Production. — Topaz is used as a gem. About 36 lb., valued 
at $2,675, was produced in the United States in ion. In the following 
year the production was valued at only $375. 

Danburite (CaB z (Si04)z) 

Danburite, which is a comparatively rare mineral, is a calcium 
borosilicate with the following theoretical composition: SK)2=48.84; 
B z 03 = 28.39 an d CaO= 22.77. Usually, however, there are present in it 
small quantities of AI2O3, Fea03, M^Oa and HzO. Thus, crystals from 
Russell, New York, contain: 

S1O2 BjOa AI2O3, etc. HzO CaO Total 

49.70 25.80 1.02 .20 23.26 99.98 

The mineral crystallizes in the orthorhombic system (rhombic bipy- 
ramidal class), with an axial ratio .5445 : 1 : .4801. Its crystals are 
usually prismatic in habit. They contain a great number of forms, of 
which wPoo(ioo), ooP66(oio), «P2(i2o), ooPJf^o), and <*>P(no) 
among the prisms, 2P4(i42), 2P2(i2i) among 
the pyramids and oP(ooi) are the most prom- 
inent (Fig. 177). The angle noAiio= 
57° 8'. 

When fresh and pure the mineral is trans- 
parent, colorless or light yellow, but when 
more or less impure is pink, honey-yellow or 
dark brown. Its streak is white, and luster 

vitreous. Its cleavage is imperfect parallel to fic. 177.— Danburite Crys- 
oP(ooi) and its fracture uneven or conchoidal. tal with » P, no (m); 
Its hardness is about 7 and density 2.95-3.02. *>Pi, j» (/); P», 101 
Its refractive indices for yellow light are: <<0; * p M" Wand 4 P«, 
«=r.63i7, (3=1-6337, 7=1-6383- ° 4 ' W ' 

Before the blowpipe the mineral fuses to a colorless glass and colors 
the 8ame green. It is only slightly attacked by hydrochloric acid, but 
after roasting is decomposed with the formation of gelatinous silica. 
It phosphoresces on heating, glowing with a red light. 

Origin. — Danburite is probably always a product of pneumatolytic 



326 DESCRIPTIVE MINERALOGY 

action, as it is found in quartz and pegmatite veins in the vicinity of 
igneous rocks and on the walls of hollows within them. 

Localities. — Its principal occurrences in this country are at Danbury, 
Conn., where it is in a pegmatite, and at Russell, N. Y., on the walls of 
rocks and hollows in a granitic rock. Its principal foreign occurrence is 
at Piz Valatscha, in Switzerland. 

EPIDOTE GROUP (Ca,R'"a(OH)(Si04),) 

The epidote group comprises six substances, of which two are di- 
morphs with thecomposition Ca2Al3(OH) (Si04) 3 = Ca2Al2(A10H) (SiQi)3. 
One of these, known as zoisite y crystallizes in the orthorhombic system, 
and the other, known as clinozoisite, in the monoclinic system. The 
other four are isomorphous with clinozoisite. These are hancockUe,- 
epidote, piedtnontite and allanite. The composition and comparative 
axial ratios of the four commoner isomorphs are as follows (assuming 
$P(Ti2) as the groundform of clinozoisite): 

Clinozoisite Ca2Al3(OH)(Si04)3 1-4457 • i • 18057 

Epidote Ca 2 (AlFe) 3 (0H)(Si04)3 15807 : 1 : 1.8057, $=64° 36' 
Piedmontite Ca 2 (Al • Mn) 3 (0H)(Si0 4 ) 3 1.6100 : 1 : 1.8326, £=64° 39' 
Allanite Ca 2 (Al-Ce-Fe) 3 (OH)(Si04)3 1.5509 : 1 : 1.7691, £=64° 59' 

Clinozoisite is rare, though its molecule occurs abundantly in iso- 
morphous mixtures with the corresponding iron molecule in epidote. 

Zoisite (Ca2Al3(OH)(Si0 4 ) 3 ) 

Zoisite is a calcium, aluminium orthosilicate containing only a small 
quantity of the corresponding iron molecule. The theoretical composi- 
tion of the pure Ca molecule is: 

SiO=39-52; Al203 = 33i92; CaO= 24.59; H20=i.97. Total = 100.00. 

Colored varieties contain a little iron or manganese. Green crystals (I), 
from Ducktown, Tenn., and red crystals (thulite) (II), from Kleppan, in 
Norway, analyze as follows: 

Si0 2 AI2O3 Fe20 3 FeO CaO MgO Mn 2 3 Na 2 H 2 Total 

I. 39.61 32.89 .91 .71 24.50 .14 2.12 100.88 

II. 42.81 31.14 2.29 ... 18.73 ••■ 1 -^3 1 &9 .64 99.13 

Zoisite crystallizes in the orthorhombic system (orthorhombic bi- 
pyramidal class), with the axial ratio .6196 : 1 : .3429. Its crystals are 



ANHYDROUS ORTHOSILICATES 



327 




m 



.-I 



^b 



usuaUy simple and without end faces. The most frequent forms are 
ooP(no), ooP4(i4o), oo P 06(010). P(iii), 2P 06 (021) and 4P 06 (041) 
are the commonest terminations (Fig. 178). The crystals are all pris- 
matic and are striated longitudinally. Their 
cleavage is perfect parallel to 00 P 06 (010). 
The angle noAiTo=63° 34'. 

The mineral is ash-gray, yellowish gray, 
greenish white, green or red in color and has a 
white streak. The rose-red variety, contain- 
ing manganese, is known as thulite. Very 
pure fresh zoisite is transparent, but the ordi- 
nary forms of the mineral are translucent. 
Its luster is glassy, except on the cleavage 
surface, where it is sometimes pearly. Its 
fracture is uneven. Its hardness is 6 and 
density about 3.3. In specimens from Duck- 
town, Term., a= 1.7002, 0= 1.7025, 7=1.7058 
for yellow light. A notable fact in connection 
with this mineral is that with increase of the 
molecule Ca2Fe3(OH)(Si04)3 in the mixture 
the plane of its optical axes tends to change 
from oP(oio) to 00 P 06 (001). 

Zoisite fuses to a clear glass before the blowpipe and gives off water, 
which causes a bubbling on the edges of the heated fragments. It is 
only slightly affected by acids, but after heating it is decomposed by 
hydrochloric acid with the production of gelatinous silica. 

Occurrence, — The mineral occurs as a constituent of crystalline 
schists, especially those rich in hornblende, or of quartz veins traversing 
them. It is also a component of the alteration product known as 
saussurite which results from the decomposition of the plagioclase 
(p. 418) in certain basic, augitic rocks known as gabbros. It is thus a 
product of metamorphism. 

Localities, — Good crystals of zoisite are found near Pregratten in 
Tyrol; at Kleppan (thulite), Parish Souland, Norway, and in the ore 
veins at the copper mines of Ducktown, Tenn., where it is associated with 
chalcopyrite, pyrite and quartz. 

Epidote (Ca 2 (Al-Fe)3(OH)(Si0 4 )3) 

Epidote, or pistazite, differs from the monoclinic dimorph of zoisite 
(clinozoisite) in containing an admixture of the corresponding iron sili- 
cate which is unknown as an independent mineral. 



Fig. i 78. — Zoisite Crystal 
with 00 P, no (m); 00 P 06 ; 
010 (6); 00 P4, 140 (/), 
2P00, 021 («) and P, in 

W- 



328 



DESCRIPTIVE MINERALOGY 



Since it consists of a mixture of an aluminium and an iron compound 
its composition necessarily varies. The four lines of figures below give 
the calculated composition of mixtures containing 15 per cent, 21 per 
cent, 30 per cent and 40 per cent of the iron molecule. 

Percent SiCk AI2O3 Fe 2 3 CaO H 2 Total 

15 38.60 28.80 6.65 24.02 1.93 100.00 

21 3823 26.76 9.32 23.78 1. 91 100.00 

30 37-67 23.71 13.31 23.43 1.88 100.00 

40 37 °4 20.32 17.75 23.04 1.85 100.00 

Most specimens contain small quantities of Mg, Fe, Mn, Na or K. 

Epidote is isomorphous with clinozoisite, crystallizing in the mono- 






Fig. 179. — Epidote Crystals with 00 P 00 , 100 (a); oP, 001 (c); P 00 , 10 1 (r); JP 00 , 

102 (1); P, 11T (») and P 00 , on (0). 







Fig. 180. — Epidote Crystals with a, c, r, f, n and as in Fig. 179. Also 00 P, 
no(m); 2P60, 2oT(/); -P66, 101 (c); —$P\, 231 (p) and |P2, 423 (/). 



clinic system (monoclinic prismatic class), with the axial ratio 1.5787 : 1 
: 1.8036. 0=64° 36'. The mineral is remarkable for its handsome 
crystals, many of which are extremely rich in forms. The crystals are 
usually columnar in consequence of their elongation parallel to the b 
axis. The most prominent forms are 00 P 60 (100) , oP(ooi), £P 66 (20T). 
Poo (ioT),P(iiT), 00 P(no) and Pob (on) (Fig. 179 and 180). In addi- 
tion to these, over 300 other forms have been identified. Twinning 
is common, with 00 Poo (100) the twinning plane. The angle no A 
iTo= 109 56'. 

Epidote is yellowish green, pistachio green, dark green, brown or, 
rarely, red. It is transparent or translucent and strongly pleochroic. 
In green varieties the ray vibrating parallel to the b axis is brown, that 
vibrating nearly parallel to c, yellow, and that vibrating perpendicular to 



ANHYDROUS ORTHOSILICATES 329 

the plane of these two is green. Its luster is glassy and its streak gray. 
Its cleavage is very perfect parallel to oP(ooi). Its hardness is 6.5 and 
density 3.3 to 3.5. The refractive indices for yellow light in a crystal 
from Zillerthal are: 0=1.7238, 13=1.7291,7=1.7343. They increase 
with the proportion of the iron molecule present, being 1.7336, 1.7593 
and 1. 7710 in a specimen containing 27 per cent of the iron epidote. 

The varieties that have been given distinct names are: 

Bucklandite, a greenish black variety in crystals that are not elon- 
gated; 

Withamite, a bright red variety containing a little MnO. 

Fragments of the mineral when heated before the blowpipe yield 
water and fuse to a dark brown or black mass that is often magnetic. 
With increase in iron fusion becomes more easy. Before fusion epidote 
is practically insoluble in acid. After heating HC1 decomposes it with 
the separation of gelatinous silica. 

The ordinary forms of the mineral are characterized by their yellow- 
ish green color, ready fusibility and crystallization. 

Occurrence and Origin. — Epidote occurs in massive veins cutting crys- 
talline schists and igneous rocks, as isolated crystals and druses on the 
walls of fissures through almost any rock and in any cavities that may 
be in them, and as the principal constituent of the rock known as epi- 
dosite. It is a common alteration product of the feldspars (p. 408), 
pyroxenes (p. 364), garnet, and other calcium and iron-bearing minerals. 
Pseudomorphs of epidote after these minerals are well known. The 
mineral is a weathering product, but is more commonly a product of 
contact and regional metamorphism. 

It has not been produced artificially. 

Localities. — Epidote crystals are so widely spread that only a few of 
the important localities in which they have been found can be mentioned 
here. Particularly fine crystals occur in the Sulzbachthal, Salzburg, 
Austria; in the Zillerthal, in Tyrol; near Zermatt, in Switzerland; in 
the Alathal, Traversella, Italy; at Arendal, Norway; in Japan, at 
Prince of Wales Island, Alaska, and at many other points in North 
America. 

Piedmontite (Ca 2 (Al-Mn) 3 (OH)(Si0 4 )3) 

Piedmontite is the manganese epidote, differing from the ordinary 
epidote in possessing manganese in place of iron. Usually, however, 
the iron and the manganese molecules are both present. Typical analy- 
ses of crystals from St. Marcel, in Piedmont, Italy (I), Otakisan, Japan 
(II), and Pine Mt., near Monterey, Md. (Ill), follow: 



330 DESCRIPTIVE MINERALOGY 

S1O2 AI2Q3 M112O3 MnO Fe2Q8 MgO CaO H2O Total 
I. 35.68 18.93 14.27 3.22 1.34 ... 24.32 2.24 100.00 
II. 36.16 22.52 6.43* 9.33 .40 22.05 .3- 2 ° 100.53* 

III. 47-37 i 8 -55 68 5 x -9 2 402 .25 15.82 2.08 100.05* 
* II. contains also .44 per cent NatO. The MniO* contained also MnO. 

III. contains also 2.03 per cent of the oxides of rare earths, .14 per cent PbO, 
.11 per cent CuO, .23 per cent Na.O and .68 per cent KsO. The specimen contained 
also a little admixed quartz, which was determined with the SiO*. 

The axial ratio of piedmontite is 1.6100 : 1 : 1.8326. 0=64° 39'. 
Its crystals are similar in habit to those of epidote, but they are .much 
simpler. The most prominent forms are 00 P «» (100), oP(ooi), P(Tn), 
$P 00(102), 00 Poo (010) and ooP(no). Twins are fairly common, 
with 00 P 60 (100) the twinning plane. 

The mineral is rose-red, brownish red or reddish black. It is trans- 
parent or translucent and strongly pleochroic in yellow and red tints 
and has a glassy luster and pink streak. It is brittle, and has a good 
cleavage parallel to oP(ooi). Its hardness is 6 and density 3.40. Its 
refractive indices are the same as those of epidote. 

Before the blowpipe piedmontite melts to a blebby black glass and 
gives the manganese reaction in the borax bead. It is unattacked by 
acids until after heating, when it decomposes in HC1 with the separation 
of gelatinous silica. 

It is characterized by its color and hardness and by its manganese 
reaction. 

Occurrence and Origin. — Piedmontite occurs as an essential constit- 
uent of certain schistose rocks that are known as piedmontite schists. 
It occurs also in veins and in certain volcanic rocks, where it is probably 
an alteration product of feldspar. Its methods of origin are the same 
as those of epidote. 

Localities. — Good crystals are found in the manganese ore veins at 
St. Marcel, Piedmont; on ilmenite in crystalline schists on the Isle of 
Groix, off the south coast of Brittany; and at a number of points on the 
Island of Shikoku, Japan, in crystalline schists and in ore veins. In 
the United States it is so abundant in the acid volcanic rocks of South 
Mountain, Penn., as to give them a rose-red color. 

Allanite (Ca 2 (Al-Ce-Fe) 3 (OH)(Si0 4 )3) 

Allanite is a comparatively rare epidote in which there are present 
notable quantities of Ce, Y, La, Di, Er and occasionally other of the 
rarer elements. Since cerium is present in the largest quantity the 



ANHYDROUS ORTHOSILICATES 



331 



formula of the mineral is usually written as above, with the under- 
standing that a portion of the cerium may be replaced by yttrium and 
the other elements. Some idea of the complex character of the mineral 
may be gained from the two analyses quoted below. The first is of 
crystals from Miask, Ural, and the second of a black massive variety 
from Douglas Co., Colo. 



I 
30.81 
16.25 
6.29 



Si0 2 

AI2O3 

Fe203 

Ce2C>3 10.13 

BeO 

Di20a 3.43 

La20a 6.35 

Y2O3 1 . 24 

FeO 8.14 

MnO 2 . 25 

MgO 13 

CaO 1043 

K 2 S3 

Na 2 

H2O 2 . 79 

CO2 



Total 98.77 



II 

3i-i3 
11.44 

6.24 

12.50 

.27 

10.98 



13-59 
.61 

.16 

9-44 
tr. 

■56 

2.78 

.21 

99.81 



Allanite rarely occurs in crystals, but when these are found they are 
usually more complex than those of piedmontite but much less compli- 
cated than those of epidote. Their axial ratio is 1.5509 : 1 : 1.7691 
with 0=64° 59'. Their habit is like that of epidote crystals. Common 
forms are 00 P 06(100), oP(ooi), <»P(iio). Twins are like those of 
epidote. The mineral usually occurs as massive, granular or columnar 
aggregates, or as ill-defined columnar crystals resembling rusty nails. 
It sometimes forms parallel intergrowths with epidote. 

It is black on a fresh fracture and rusty brown on exposed surfaces, 
and has a greenish gray or brown streak. It has a glassy luster and is 
translucent in thin splinters, with greenish gray or brownish tints and 
is pleochroic in various shades of brown. Its hardness is 5-6 and 
density 3-4, both varying with freshness and composition. The cleav- 
ages are imperfect and the fracture uneven. Its indices of refraction 
are nearly the same as those of epidote. 






32 DESCRIPTIVE MINERALOGY 



Small fragments of fresh allanite fuse to a blebby black magnetic 
glass before the blowpipe and are decomposed by HC1 with the separa- 
tion of gelatinous silica. 

Allanite is distinguished by its color, manner of occurrence, and the 
reaction for water in the closed tube. 

The mineral alters readily on exposure to the weather, yielding 
among other compounds mica and limonite. 

Occurrence. — Allanite occurs as an original constituent in some 
granites, and other coarse-grained rocks. It is found also in gneisses, 
occasionally in volcanic rocks and rarely as a metamorphic mineral in 
crystalline limestones. 

Localities. — The best crystals have been found in the druses of a 
volcanic rock at Lake Laach, Prussia; in coarse-grained granitic rocks 
at several places in the Tyrol; in the limestone at Pargas, Finland; and 
at various points in Ural, Russia. Massive allanite occurs in the coarse 
granite veins at Hittero, Norway and as the constituents of granites 
at many places in the United States. Parallel intergrowths with epidote 
are found in granite at Ilchester, Md. 

CHONDRODITE GROUP 

The chondrocyte group of minerals includes four members of the 
general formula (Mg(F • OH^Mg^SiC^)* in which x equals i, 3, 5, 7, and 
y, 1, 2, 3, 4. Of these, one (humite) may be orthorhombic. The other 
three are monoclinic with the angle £=90°. The four members of the 
group with their compositions and axial ratios are: 

ProlectiU (Mg(F-OH) 2 )Mg(Si0 4 ) 1.0803 : 1 : 1.8862 £=90° 
Chondrodite (Mg(F-OH) 2 )Mg3(Si0 4 ) 2 1.0863 : 1 : 3-1445 £=90 

b Z 
Humite (Mg(F-OH) 2 )Mg5(Si0 4 )3 1.0802 : 1 : 4.4033 

Clinohumite (Mg(F-OH) 2 )Mg 7 (Si0 4 ) 4 1.0803 : 1 : 5.6588 0=90 

To show the similarity in the ratios between the lateral axes of the 
four minerals, the & axis of humite is written as 1. Chondrodite, humite 
and clinohumite frequently occur together. Chondrodite has been 
reported at more localities than either humite or clinohumite, but it is 
not certain that much of it is not clinohumite. The three minerals 
resemble one another very closely. They are relatively unstable under 
conditions prevailing at moderate depths in the earth's crust, passing 
easily into serpentine, brucite or dolomite. Only chondrodite is de- 
scribed. 



ANHYDROUS ORTHOSILICATES 



333 



Chondrodite (Mg3(Mg(FOH) 2 )(Si0 4 )2) 

Chondrodite is a rather uncommon mineral that occurs mainly as a 
constituent of metamorphosed limestones that have been penetrated 
by gases and water emanating from igneous rocks. It is a characteris- 
tic contact mineral. 

Its composition varies somewhat in consequence of the fact that OH 
and F possess the power to mutually replace one another. The two 
analyses below are typical of varieties containing a maximum amount 
of F. 



Si0 2 


MgO 


FeO 


H 2 


I- 33 • 77 


57.98 


3-9 6 


i-37 


H. 35 42 


54-22 


5-72 


• • • • 



F F=0 Total 

5.14=102.22 — 2.16 100.06 
9 . 00= 104 . 36 — 3 . 78 100 . 58 

I. Crystals from limestone inclusions in the lava of Vesuvius. 
II. Grains separated from the limestone of the Tilly Foster Iron Mine, Brewster, 

N. Y. 

Chondrodite is monociinic (prismatic class), with an axial ratio 
1.0863 : * : 3 I 445- = 9O O - The crystals vary widely in habit and 
are often complex. The forms oP(ooi), 
00 P * (100), 00 P 00 (010) and various unit 
and clinobemipyramids of the general sym- 
bol xPi are frequently present, but other 
forms are also common (Fig. 181). Twin- 
ning about oP(ooi) is also common. 
Usually, however, the mineral occurs in 
little rounded grains, in some instances 

showing crystal faces, scattered through Fig. 181.— Chondrodite Crys- 
tal with oP, 001 (c); JP 5b , 

012 (0; |F2,l27(fi);fP2, 
"5 (*); fP2, 123 (r,); 
-2P2, 121 (r 4 ); -P, in 
(*); P, In (-»i); JP 00, 

103 M; P«, 101 (-*) 
and -P5o, 101 («j). The 
a axis runs from right to left 
and the upper left hand 
octant is assumed to be 
minus. 




limestone. 

When fresh, chondrodite has a glassy 
luster, is translucent and is white or has a 
light or dark yellow, brown or garnet color. 
It has a distinct cleavage parallel to oP(ooi), 
a conchoidal fracture, a hardness of 6 and 
a density of 3.15. Its refractive indices 
for yellow light are: a= 1.607, 0=1.619, 
7=1.639. 

Before the blowpipe chondrodite bleaches 
without fusing. With acids it decomposes with the production of 
gelatinous silica. 



334 



DESCRIPTIVE MINERALOGY 



It weathers readily to serpentine, chlorite and brucite, and conse- 
quently many grains are colored dark green or black. 

Occurrence. — Chondrodite, as has been stated, occurs in meta- 
morphosed limestones. It also occurs in sulphide ore bodies and in a 
few instances in magnetite deposits. It is probably in all cases a pneu- 
matolytic or metamorphic product. 

Localities. — It is found as crystals in the blocks enclosed in the lavas 
of Vesuvius; in the copper mines of Kapveltorp, Sweden; in limestone 
in the Parish of Pargas, Finland; and at the Tilly Foster Iron Mine, at 
Brewster, N. Y. It occurs as grains in the crystalline limestone of 
Sussex Co., N. J., and Orange Co., N. Y. 



DATOLITE GROUP 

The members of the datolite group are four in number, but 
of these only two, viz., datolite (Ca(B'OH)Si04) and gadolinite 
(Be2Fe(YO)2(Si0.i)2) are of sufficient importance to be described here. 
Both minerals crystallize similarly in the monoclinic system (mono- 
clinic prismatic class), with axial ratios that are nearly alike. 

Datolite alb: ^=.6345 : 1 : 1.2657 /S = 8o°5i' 
Gadolinite alb: £ = .6273 : 1 : 1.3215 /S = 8o°26i' 

Datolite (Ca(BOH)Si0 4 ) 

Datolite, or datholite, is characteristically a vein mineral. 

The composition corresponding to the 
formula given above is: 

^ SiO=37.S4; B 2 0:* = 21.83; CaO=35.oo; 

1120=5.63. Total= 100.00 




Some specimens contain a little AI2O3 and 
Fe203 but, in general, crystals that have 
been analyzed give results that are in 
close accord with the theoretical com- 

Fig. 182 —Datolite Crystal with P° sltl0n - 

oop 56, 100 (a); 00 p, no (m); The mineral crystallizes in fine crys- 

-P co , 101, (<*>); — JP co , 102 tals that are often very complicated (Fig. 

(*); -P, in (»); -P2, 212 !8 2 ). About 115 different forms have 

(*); P * , on (m z ) and JP <2 , been observed on them . Because of the 

suppression of some faces by irregular 
growth many of the crystals are columnar in habit, others are tabular. 
Most crystals, however, are nearly equi-dimensional. The angle 



ANHYDROUS ORTHOSILICATES 335 

noA 110=64° 40'. The mineral occurs also in globular, radiating, 
granular and massive forms. 

Datolite is colorless or white, when pure, and transparent. Often, 
however, it is greenish, yellow, reddish or violet, and translucent. Its 
streak is white and its luster glassy. It has no distinct cleavage. Its 
fracture is conchoidal. Its hardness is 5 and its sp. gr. about 3. Some 
crystals are pyroelectric. For yellow light, 0=1.6246, 18=1.6527, 
7=1.6694. 

Before the blowpipe it swells, and finally melts to a clear glass and, 
at the same time, it colors the flame green. Its powder before heating 
reacts strongly alkaline. After heating this reaction is weaker. The 
mineral loses water when strongly heated, and yields gelatinous silica 
when treated with hydrochloric acid. 

The mineral is characterized by its crystallization, its easy fusibility 
and the flame reaction for boron. 

Synthesis. — Datolite has not been produced artificially. 

Occurrence, Origin and Localities. — It occurs on the walls of clefts 
in igneous rocks, in pegmatite veins and associated with metallic com- 
pounds in ore veins. It is found in many ore deposits of pneumatolytic 
origin, notably at Andreasberg in the Harz Mts.; at Markirch, in 
Alsace; in the Seisser Alps, in Tyrol; in the Serra dei Zanchetti in the 
Bolognese Apennines; at Arendal, Norway, and at many other places. 
In North America it occurs at Deerfield, Mass.; at Tariffville, Conn.; 
at Bergen Hill, N. J.; and at several points in the copper districts of 
the Lake Superior region. 

Gadolinite (Be 2 Fe(YO) 2 (Si0 4 )2) 

Gadolinite is a rather rare mineral with a composition that is not 
well established. Its occurrence is limited to coarse granite veins or 
dikes — pegmatites — of which it is sometimes a constituent. 

Its theoretical composition is as follows, on the assumption that it is 
analogous to that of datolite: 

SiO= 25.56; ¥203=48.44; FeO= 15.32; BeO= 10.68. Total=ioo.oo, 
but nearly all specimens contain cerium oxides. Others contain nota- 
ble quantities of erbium or lanthanum oxides and small quantities of 
thorium oxide. Nearly all show the presence of Fe203, AI2O3, CaO and 
MgO, and in some helium has been found. 

The mineral is found massive and in rough crystals with an axial 
ratio alb: £=.6273 : 1 : 1.3215. £=89° 263'. The crystals show 
comparatively few forms, of which oop(no), oP(ooi), Poo (on), 



336 DESCRIPTIVE MINERALOGY 

£P 00(012), P(Tii) and — P(in) are the most common. They are 
usually columnar in habit and are rough and coarse. The angle 
iioAiTo=64° 12'. 

Gadolinite is usually black or greenish black and opaque or trans- 
lucent, but very thin splinters of fresh specimens are translucent or 
transparent in green tints. Its luster is glassy or resinous, streak 
greenish gray and fracture conchoidal. Its hardness is 6-7 and its 
density about 4-4.5. Upon heating the density increases. Many crys- 
tals appear to be made up of isotropic and anisotropic substance, and 
some to consist entirely of isotropic matter. This phenomenon has 
been explained in a number of different ways, but no one is entirely satis- 
factory. In general, the isotropic material is believed to be an amor- 
phous alteration form of the anisotropic variety. It may be changed 
into the anisotropic form by heating. 

The crystallized gadolinite swells up in the blowpipe flame without 
becoming fused and retains its transparency. The amorphous variety 
also swells without melting, but yields a grayish green translucent mass. 
The mineral phosphoresces when heated to a temperature between that 
of melting zinc and silver. After phosphorescing it is unattacked by 
hydrochloric acid. Before heating it gelatinizes with the same reagent. 
The mineral is weakly radioactive. 

Localities and Origin. — Gadolinite occurs in the pegmatites of Ytterby 
near Stockholm, and of Fahlun, Sweden; on the Island of Hittero, in 
southern Norway; in the Radauthal, in Harz; at Barringer Hill, Llano 
Co., Texas, as nodular masses and large rough crystals; and at Devil's 
Head, Douglas Co., Colo. In the last locality it occurs in a de- 
composed granite as a black isotropic variety with a very complex 
composition. Specimens analyzed as follows: 



I n 

SiC>2 22.13 21.86 

ThCfe 89 .81 

AI2O3 2.34 .54 

Fe 2 03 1 . 13 3-59 

Ce203 1 1. 10 6.87 

(La-Di)203 21.23 19.10 

Y2O3 9-5° 12.63 

Er203 12.74 15.80 



i n 

FeO 10.43 1136 

BeO 7.19 5.46 

CaO 34 .47 

H 2 86 .74 

Other .60 .79 



Total 100.48 100.02 



It has apparently in some cases solidified from an igneous magma. 
In others it is of pneumatolytic origin. 



ANHYDROUS ORTHOSILICATES 



337 



Staurolite (Fe(A10H)(A10) 4 (Si0 4 )2) 

Staurolite is a mineral that is interesting from the fact that it fre- 
quently forms twinned crystals that resemble a cross in shape, and which 
consequently, during the Middle Ages, was held in great veneration. 
Its composition is not well established. The composition indicated by 
the formula above is as shown in the first line below (I). Three analyses 
are quoted in the next three lines: 





Si0 2 


AI2O3 


Fe20s 


FeO 


MgO 


H 2 


Ti02 


Total 


I. 


26.3 


55-9 


■ • • * 


158 


• • • • 


2.00 


• ■ ■ 


100.00 


II. 


27-38 


5420 


6.83 


913 


■ • ■ • 


i-43 


• • • 


98.97 


III. 


3°- 2 3 


51.16 


• ■ • • 


14.66 


2-73 


1.26 


.29 


100.33 


IV. 


27.91 


52.92 


6.87 

• • 


7.80 


328 


i-59 


• ■ • 


100.37 



I. Theoretical composition. 
II. From Monte Campione, Switzerland. 

III. From Morbihan, France. 

IV. From Chesterfield, Mass. 

Staurolite crystallizes in the orthorhombic system (bipyramidal 
class) in simple crystals with the axial ratio .4734 : 1 : .6828. The indi- 






Fic. 183. Fig. 184. Fig. 185. 

Fig. 183. — Staurolite Crystal with 00 P, no (tn); wPoo, 100 (6); oP, 001 (c) and 

Poo , 101 (r). 

Fig. 184. — Staurolite Crystal Twinned about fP 00 (032). 

Fig. 185. — Staurolite Crystal Twinned about |P| (232). 

vidual crystals are usually bounded by 00 P(i 10), 00 P 60 (001), P 60 (101) 
and often oP(ooi), but all their faces are rough (Fig. 183). The angle 
1 10 Alio =50° 40'. More common, however, than the simple crystals 
are interpenetration twins. The most common of these are of two kinds, 
(1) with fP 06 (032) the twinning plane (Fig. 184), and (2) with ^£(232) 
the twinning plane (Fig. 185). Both types of twins yield crosses, but 
the arms of the first type are perpendicular to one another and those of 



338 DESCRIPTIVE MINERALOGY 

the second type make angles of about 6o° and 120 . Sometimes the 
twinning is repeated, giving rise to trillings. 

The mineral is reddish or blackish brown, and has a glassy or greasy 
luster. Its streak is white. It is slightly translucent in fresh crystals, 
but usually is opaque. In very thin pieces it is pleochroic in hyacinth- 
red and golden yellow tints. Its cleavage is distinct parallel to 00 P 06 
(010) and indistinct parallel to ooP(no). Its fracture is conchoidal, 
its hardness 7 and its density 3.4-3-8. For yellow light, a= 1.736, 
18=1.741, 7=1.746. 

Before the blowpipe staurolite is infusible, unless it contains man- 
ganese, in which case it fuses to a black magnetic glass. It is only 
slightly attacked by sulphuric acid. 

It is distinguished from other minerals by its crystallization, in- 
fusibility and hardness. 

Staurolite weathers fairly readily into micaceous minerals, such as 
chlorite (p. 428) and muscovite (p. 355). 

Synthesis. — It has not been produced in the laboratory. 

Occurrence. — The mineral occurs principally in mica schist and other 
schistose rocks where it is the result of regional or contact metamor- 
phism. Because of its method of occurrence it frequently contains 
numerous mineral inclusions, among them garnet and mica. 

Localities. — Good crystals of staurolite are found in the schists at 
Mte. Campione, Switzerland; in the Zillerthal, Tyrof; at AschafFen- 
burg, in Bavaria; at various places in Brittany, France; and in the 
United States, at Windham, Maine, at Franconia, N. H., at Chester- 
field, Mass., in Patrick Co., Va., and in Fannin Co., N. C. 

Uses. — Twins of staurolite are used, to a slight extent, as jewelry. 
Specimens from Patrick Co., Virginia, are mounted and worn as charms 
under the name of " Fairy Stones." 

Dumortierite (Al(A10) 7 H(BO)(Si0. 4 ) 3 ) 

Dumortierite is one of the few blue silicates known. It is a borosili- 
cate with a composition approaching the formula indicated above. The 
analysis of a sample from Clip, Arizona, gave (I) : 

Si0 2 AI2O3 Fe 2 3 Ti 2 3 MgO B2O3 P2O5 Loss on Ign. Total 

I.27.99 64.49 ••• tr 4.95 .20 1.72 99.35 

II.28.58 63.31 .21 1.49 5.21 ... 1.53 100.33 

Specimens from California (II) contain in addition notable quantities 
of Ti0 2 , which is thought to exist as Ti 2 03 replacing a part of the AI2O3. 



ANHYDROUS ORTHOSILICATES 339 

The mineral crystallizes in the orthorhombic system in aggregates of 
fibers, needles or very thin prisms exhibiting only ooP(no) and 
oo P 56 (ioo) without end faces. Its axial ratio is a : £=.5317 '• 1, and 
the prismatic angle no A 1^0=56°. Its crystals possess a distinct 
cleavage parallel to 00 P 60 (100) and a fracture perpendicular to the 
long axes of the prisms. Twinning is common, with ooP(no) the 
twinning plane. 

Dumortierite is commonly some shade of blue, but in some cases is 
green, lavender, white, or colorless. It is translucent or transparent 
and strongly pleochroic, being colorless and red, purple or blue. Its 
streak is light blue. Hardness is 7 and density 3.3. Its refractive indices 
for yellow light are: a= 1.678, /3= 1.686, 7= 1.089. 

Before the blowpipe the mineral loses its color and is infusible. It is 
insoluble in acids. 

It is distinguished from other blue silicates by its fibrous or columnar 
character and its insolubility in acids. 

Its principal alteration products are kaolin and damourite 
(pp. 404, 357). 

Occurrence and Localities. — Dumortierite occurs only as a constit- 
uent of gneisses and pegmatites. It is found in pegmatite near Lyons, 
France; near Schmiedeberg, in Silesia; at Harlem, N. Y.; in a granular 
quartz, at Clip, Yuma Co., Ariz., and in a dike rock composed of quartz 
and dumortierite, near Dehesa, San Diego Co., Cal. It is evidently 
a pneumatolytic mineral. Its common associates are kyanite, anda- 
lusite or sillimanite. 

SODALITE GROUP 

The sodalite group includes a series of isometric minerals that may be 
regarded as compounds of silicates with a sulphate, a sulphide or a chlor- 
ide, or, perhaps better, as silicates in which are present radicals con- 
taining CI, SO4 and S. The minerals comprising the group are haiiynite, 
nosean, sodalite and lasurite. Of these, sodalite appears to be a mixture 
of 3NaAlSi04 and NaCl, in which the CI has combined with one atom of 
Al, thus Na4(ClAl)Al2(Si04)3- The other members of the group are 
comparable with this on the assumption that the CI atom is replaced by 
the radicals NaSC>4, and NaSa. It is possible, however, that all are 
molecular compounds as indicated by the second set of formulas given 
below. All are essentially sodium salts, except that in typical haiiynite 
a portion of the Na is replaced by Ca. The chemical symbols of the 
four minerals with the calculated percentages of silica, alumina and 
soda corresponding to their formulas are: 



340 



DESCRIPTIVE MINERALOGY 



Si0 2 
37H 



AI2O3 
31.60 

27.03 

27.32 

26.9 



NasO 

25 .60 

27.26 

16.53 
27-3 



Sodalite Nai(Cl • Al) AWSiQOs, or 

3NaAlSi0 4 -NaCl 
Noselite Na4(NaS04 • Al) Al 2 (Si04)3, or 31 . 65 

3NaAlSi0 4 • Na 2 S04 
Hatiynite (Na2Ca)2(NaS04* Al)Al2(SiOi)3, or 31.99 

3NaAlSi0 4 • CaS0 4 
Lasurite Na4(NaS3 • Al)Al2(Si04)3, or 31.7 

3 NaAlSi04-Na 2 S-S* 

SodaUte (Na4(Cl-Al)Al 2 (Si0 4 )3) 

Sodalite, theoretically, is the pure sodium compound corresponding 
to the composition indicated by the formula given above. Natural 
crystals, however, usually contain a little potassium in place of some of 
the sodium and often some calcium, as indicated by the analyses of 
material from Montreal, Canada (I), and Litchfield, Maine (II), quoted 
below. Moreover, their content of CI is not constant. 



SiCfe AI2O3 Na 2 K 2 OCaO CI C1=0 

I- 37-52 31-38 25.15 .78 .35 6.91 = 102.09 -I-S5 

II- 37-33 31-87 24.56 .10 ... 6.83 - 101.76* -1.54 

* Includes 1.07 per cent H*0. 



Total 
100.54 
100.22 




Sodalite occurs massive and in crystals that appear to be holohedral, 
but etch figures indicate that they are probably tetrahedrally hemi- 

hedral (hextetrahedral class). Most crystals 
are dodecahedral in habit, though some are 
tetrahexahedral and others octahedral. The 
forms usually developed are ooO(no), 
00O00 (100), O(iii), 202(112) and 404(114). 
Interpenetration twins of two dodecahedrons 
are common, with O the twinning plane (Fig. 
186). These often possess an hexagonal habit. 
The mineral is colorless, white or some 
light shade of blue or red, and its streak is 
white. Its luster is vitreous. It is trans- 
parent, translucent and sometimes opaque. 
Its cleavage is perfect parallel to ooO(no) 
and its fracture conchoidal. Its hardness is 5-5.6, and its density 
2.3. Its refractive index for yellow light, n= 1.4827. Some specimens 
are distinctly fluorescent and phosphorescent. 



Fig. 186. — Sodalite. Inter- 
penetration Twin of Two 
Dodecahedrons Elon- 
gated in the Direction of 
an Octahedral Axis and 
Twinned about O(in). 



ANHYDROUS ORTHOSILICATES 341 

Before the blowpipe, colored varieties bleach and all varieties swell 
and fuse readily to a colorless blebby glass. The mineral dissolves com- 
pletely in strong acids and yields gelatinous silica, especially after heat- 
ing. When dissolved in dilute nitric acid its solution yields a chlorine 
precipitate with silver nitrate. Its powder becomes brown on treatment 
with AgNOa, in consequence of the production of AgCl. 

The mineral is best distinguished from other similarly appearing 
minerals by the production of gelatinous silica with acids and the reac- 
tion for chlorine. 

As a result of weathering sodalite loses CI and Na and gains water. 
Its commonest alteration products are zeolites (p. 445), kaolin (p. 440), 
and muscovite (p. 355). 

Syntheses. — It has been produced artificially by dissolving nepheline 
powder in fused sodium chloride, and by decomposing muscovite 
with sodium hydroxide and NaCl at a temperature of 500 C. 

Occurrence and Origin. — Sodalite occurs principally as a constituent 
of igneous rocks rich in alkalies and as crystals on the walls of pores in 
some lavas. It is also known as an alteration product of nepheline. 

Localities. — Good crystals are found in nepheline syenite at Ditr6, 
in Hungary, in the lavas of Mte. Somma, Italy; in the pegmatites of 
southern Norway; and at many other points where nepheline rocks 
occur. In North America it is abundant in the rocks at Brome, near 
Montreal; in the Crazy Mts., Montana, and at Litchfield, Maine. The 
material at the last-named locality is light blue. 

Noselite and Haiiynite ((Na.Ca) 2 (NaS0 1 Al)Al 2 (Si0. 1 )3) 

Noselite, or nosean, and haiiynite, or haiiyn, consist of isomorphous 
mixtures of sodium and calcium molecules of the general formula given 
above. Those mixtures containing a small quantity of calcium are 
usually called nosean, while those with larger amounts constitute hatiyn. 
The theoretical nosean and haiiyn molecules are indicated on p. 340. 
The theoretical compositions of the pure nosean molecule (I) and of the 
most common hatiyn mixture (II) are as follows: 

Si02 AI2O3 Fe 2 3 CaO Na 2 Ka 2 SO3 H 2 Total 

27.26 .... 14.06 .... 100.00 

9.94 16.53 .... 14.22 .... 100.00 

.31 .21 20.91 .... 10.58 1.63 99.61* 

10.08 13.26 3.23 12.31 100.08 

* Contains also .57 per cent CI. 



I. 


31-65 


27.03 


II. 


31 99 


2732 


III. 


35-99 


29.41 


IV. 


33.78 


27.42 



342 DESCRIPTIVE MINERALOGY 

In line III is the analysis of a blue nosean from Siderao, Cape Verde, 
and in line IV, the analysis of a blue haiiyn from the lava of Monte Vul- 
ture, near Melfi, Italy. 

Nosean and haiiyn are isomorphous with sodalite. They crystallize 
is the isometric system in simple combinations with a dodecahedral 
habit. The principal forms observed are ooO(no), ooOoo(ioo) 
0002(102), O(ni) and 202(112). Contact and interpenetration twins 
are common, with O(in) the twinning plane. The twins are often 
columnar. 

The minerals have a glassy or greasy luster, are transparent or trans- 
lucent, have a distinct cleavage parallel to ooO(no) and an uneven or 
conchoidal fracture. Their hardness is 5.6 and density 2.25 to 2.5, the 
value increasing with the amount of CaO present. Nosean is generally 
gray and haiiyn blue, but both minerals may possess almost any color, 
from white through light green and blue tints to black. Red colors are 
rare. The streaks of both minerals are colorless, or bluish. For yel- 
low, light n= 1.4890 to 1.5038, increasing with increase in the Ca 
present. Both minerals are fluorescent and phosphorescent. 

Before the blowpipe both minerals fuse with difficulty to a blebby 
white glass, the blue haiiyn retaining its color until a high temperature 
is reached. In this respect it differs from blue sodalite which bleaches 
at comparatively low temperatures. Upon treatment with hot water 
both minerals yield Na2S04. They are decomposed with acids yielding 
gelatinous silica. The powders of both minerals react alkaline. Both 
also give the sulphur reaction with soda on charcoal. 

The minerals are easily distinguished from all others by their crys- 
tallization, gelatinization with acids and reaction for sulphur. 

Both minerals upon weathering yield kaolin or zeolites and 
calcite. 

Synthesis. — Crystals of noselite have been made by melting together 
Na2COa, kaolinite and a large excess of Na2SO*. 

Occurrence. — Haiiyn and nosean occur in many rocks containing 
nepheline, especially those of volcanic origin and in a few metamorphic 
rocks. Haiiyn is so common in some of them as to constitute an essen- 
tial component. 

Localities. — Both minerals are found in good crystals in metamor- 
phosed inclusions in the volcanic rocks of the Lake Laach region, in 
Prussia; also in the rocks of the Kaiserstuhl, in Baden; in those of 
the Albanian Hills, in Italy, and at S. Antao in Cape Verde. In 
America haiiyn has been reported from the nepheline rocks of the 
Crazy Mts., Montana. 



ANHYDROUS ORTHOSILICATES 343 

Lasurite (Na^NaSa- Al)Al 2 (Si0 4 )3) 

Lasurite is better known as lapis lazuli. It is bright blue in color 
and was formerly much used as a gem stone. The material utilized for 
gem purposes is usually a mixture of different minerals, but its blue 
color is given it by a substance with a composition corresponding to the 
formula indicated above. Since the artificial ultramarine, which is 
ground and used as a pigment, also has this composition, the molecule is 
sometimes represented by the shortened symbol US3, or if it contains 
but two atoms of S, by the symbol US2. The deep blue lasurite from 
Asia contains as its coloring material a substance with a composition 
that may be represented by 15.7 molecules of US3, 76.9 molecules of 
hatiyn and 7.4 molecules of sodalite, corresponding to the percentages: 



SiCb 


AI2O3 


CaO 


Na20 K 2 


3 2 -5 2 • 


27.61 


6.47 


19.45 .28 


SOa 


S 


CI 


Total (Less CI = 0) 


10.46 


2.71 


•47 


99.97 = 99.42 



Lasurite is thus the name given to the blue coloring matter of lapis 
lazuli, which is a mixture. It apparently crystallizes in dodecahedrons. 
Its streak is blue, its cleavage is dodecahedral, its hardness about 5 and 
its specific gravity about 2.4. Before the blowpipe it fuses to a white 
glass. Its powder bleaches rapidly in hydrochloric acid, decomposes 
with the production of gelatinous silica and yields H2S. 

It is distinguished from blue sodalite and hatiyn by the reaction with 
HC1, especially by the evolution of H2S. 

Occurrence. — Lasurite is principally a contact mineral in limestone. 

Localities. — Good lapis lazuli occurs at the end of Lake Baikal, in 
Siberia; in the Andes of Ovalle, in Chile; in the limestone inclusions in 
the lavas of Vesuvius, and in the Albanian Mts., Italy. 

Uses. — Lapis lazuli is used as an ornamental stone in the manufacture 
of vases, and various ornaments, in the manufacture of mosaics, and as a 
pigment, when ground, under the name ultramarine. Most of the ultra- 
marine at present in use, however, is artificially prepared. 

ACID ORTHOSILICATES 

Prehnite Q^CazAfeCSiO^a) 

Prehnite is nearly always found in crystals, though it occurs also in 
stalactitic and granular masses. 

The theoretical composition of the pure mineral is Si02 = 43.69, 




344 DESCRIPTIVE MINERALOGY 

Al 2 03 = 24.78, CaO= 27.16, and H20=4-37- Most crystals, however, 
contain small quantities of Fe203 and other constituents. 

SiO, A1,0, Fe«Oa FeO CaO MgO HaO Total 

Jordansmtthl, Silesia 44. 12 26.00 .61 25.26 tr 4.91 100.90 

Cornwall, Penn 42.40 20.88 5.54 27.02 tr 4.01 99-85 

Chlorastrolite, Isle Royale 3741 24.62 2.21 1.81 22.20 3.46 7.72 99-75* 

* Also .32 per cent NajO. 

Its crystallization is orthorhombic and hemimorphic (rhombic py- 
ramidal class), with a: b: c=. 8420: 1 : 1.1272. The crystals vary 
widely in habit, but they contain comparatively few forms. The most 
prominent are oP(ooi), ooP(no), 6Poo(o6i), 2P(22i) and 6P(66i) 

(Fig. 187). The angle noAiTo=8o° 
12'. Because they exhibit pyroelectric 
polarity in the direction of the a axis the 

, . , crvstals are thought to be twins, with 
Fig. 187. — Prehnite Crystal with "t» - / \ .1 A • • 1 

' , ,. n - / x 00 P 00 (100) as the twinning plane. 

00 P, no (m); 00 poo , 100 (a); v ' & v 

iP«, 304 (»); iP*, 308 (») Cleavage is good parallel to oP(ooi). 
and oP, 001 (c). The crystals are frequently tabular 

parallel to oP(ooi), although other 
habits are also common. Isolated individuals are rare, usually many 
are grouped together into knotty or warty aggregates. 

Prehnite is colorless or light green, and transparent or trans- 
lucent, and it has a colorless streak. Its luster is pearly on oP(ooi) but 
glassy on other faces. Its fracture is uneven, its hardness 7+ and its 
density 2.80-2.95. For yellow light, a= 1.616, 0= 1.626, 7 = 1.649. 

Before the blowpipe prehnite exfoliates, bleaches and melts to a 
yellowish enamel. At a high temperature it yields water. Its powder is 
strongly alkaline. It is partially decomposed by strong hydrochloric 
acid with the production of pulverulent silica. After fusion it dissolves 
readily in this acid yielding gelatinous silica. 

The mineral has not been produced artificially. 

Occurrence. — Prehnite occurs as crystals implanted on the walls of 
clefts in siliceous rocks, in the gas cavities in lavas, and in the gangue of 
certain ores, especially copper ores. It is found also as pseudomorphs 
after analcite (p. 458), laumonite (p. 451), and natrolite (p. 454). In 
all cases it is probably a secondary product. 

Localities. — Fine crystals come from veins at Harzburg, in Thiiringia; 
at Striegau and Jordansmiihl, Silesia, and at Fassa and other places in 
Tyrol. Good crystals are found also in the Campsie Hills in Scotland. 
The mineral is abundant in veins with copper along the north shore of 



ANHYDROUS ORTHOSILICATES 345 

Lake Superior and on Keweenaw Point, and it occurs also at Farmington, 
Conn.; Bergen Hill, N. J. ; and Cornwall, Penn. 

Uses. — The mineral known as chlorastrolite is probably an impure 
prehnite. It is found on the beaches of Isle Royale and the north shore 
of Lake Superior as little pebbles composed of stellar and radial bunches 
of bluish green fibers. The pebbles were originally the fillings of gas 
cavities in old lavas. They are polished and used, to a slight extent, as 
gem-stones. About $2,000 worth were sold in 191 1 and $350 worth in 
1912. 

Arinite (H(Ca-Fe-Mn)3Al 2 B(Si0 4 )4) 

Axinite is especially noteworthy for its richness in crystal forms. 
The mineral is a complicated borosilicate for which the formula given 
above is merely suggestive. Analyses of crystals from different localities 
vary so widely that no satisfactory simple formula has been proposed 
for the mineral. Four recent analyses are quoted below: 

Kadauthal Stricgau Oisans Cornwall 

SiCfe 39 26 42.02 41 .53 42.10 

B2O3 4-91 5.00 4.62 4.64 

AI2O3 1446 17-73 *7 °o I7-40 

FeoOa 2.62 .93 3.90 3.06 

FeO 3.65 6.55 4.02 5.84 

MnO 2.80 6.52 3.79 4.63 

CaO 29.70 19.21 21.66 2 °S3 

MgO 2 . 00 .38 .74 .66 

H2O 1.22 1.77 2.16 1.80 



Total.... 100.62 100. 11 100.32 100.66 

Axinite crystallizes in the triclinic system (pinacoidal class), with 
a: b : £=.4921 : 1 : .4797 and a=82° 54', 0=91° 52', 7=131° 32'. 
The crystals are extremely varied in habit but nearly all are somewhat 
tabular parallel to 'P(iii), ooP'(no) or 00 'P(iTo). About 45 forms 
have been observed. In addition to the three mentioned, 2'P' 06 (201), 
P'(ui), /P(iFi), 2/P' 06 (021), 00 P 06 (010) and 00 P 00 (100) are the most 
frequently met with (Figs. 188, 189). The plane 'P(iTi) is usually 
striated parallel to its intersection with 00 'P(iTo). The angle 100 A 1T0 
= 15° 34'. The cleavage is indistinct parallel to ooP'(no) and the 
crystals are strongly pyro electric. 

Axinite is brownish, gray, green, bluish or pink, and is strongly pleo- 
chroic in pearl-gray, olive-green and cinnamon-brown tints. It is 



346 



DESCRIPTIVE MINERALOGY 



transparent or translucent and has a glassy luster and a colorless streak. 
Its fracture is conchoidal or uneven. It is brittle, has a hardness of 6-7 
and a density of 3.3. For red light, a= 1.6720, 0= 1.6779, 7 = 1.6810. 

Axinite, before the blowpipe, exfoliates and fuses to a dark green 
glass which becomes black in the oxidizing flame. It colors the flame 
green, especially upon the addition of KHSO4 and CaF2 to its powder. 
Its powder reacts alkaline. It is only slightly attacked by acids. After 




Fig. 188. 



Fig. 189. 




Fig. 188. — Axinite Crystal with 00 Poo, 100 (a); 2'P'oo, 201 (5); 00 p/ f no (f»); 

00 ,'P, 1T0 (M); P', in (*) and 'P, 1T1 (r). 
Fig. 189. — Axinite Crystal with M , m, a, r and s as in Fig. 188. Also 00 P 00 , 

010 (b); 2P' » , 021 (y); ,P, In (e); §^3, 132 (<?); 4,P*5, 241 (0); 3JP3, T31 (F); 

00 /FX x 30 (w); 3'PX 131 (») and 4'Pl, 241 (d). 

fusion, however, it dissolves readily with the production of gelatinous 
silica. 

The mineral is easily characterized by its crystallization and the 
green color it imparts to the flame. 

It has not been produced artificially. 

Pseudormorphs of chlorite after axinite have been found in Dart- 
moor, England. 

Occurrence. — Axinite crystals occur in cracks in old siliceous rocks. 
It is found also in ore veins and. as a component of a contact rock com- 
posed mainly of augite, hornblende and quartz, occurring near the 
peripheries of granite and diabase masses. It was formed by the aid 
of pneumatolytic processes. 

Localities. — Excellent crystals of axinite are found at Andreasberg 
and other places in the Harz Mts.; near Striegau, in Silesia; near 
Poloma, in Hungary; at the Piz Valatscha, in Switzerland; near Vernis 
and at other points in Dauphine, France; at Botallak, Cornwall, Eng- 
land; at Konigsberg, Norway; Nordmark, Sweden; Lake Onega, and 
Miask, Russia; at Wales in Maine and at South Bethlehem, Penn. 




ANHYDROUS ORTHOSILICATES 347 



Dioptase (H 2 CuSi0 4 ) 

Dioptase is especially interesting because of its crystallization, which 
is rhombohedral tetartohedral (trigonal rhombohedral class). Its crys- 
tals are columnar. Their axial ratio is i : .5342. They are usually 

bounded by 00 P2(ii2o), — 2R(o22i) or R(ioii) and — — — - (1341) or 

4P4 r - * r 

+ (3141) (a rhombohedron of the third order, Fig. 190). Besides 

4 J 

occurring as crystals the mineral is found also 
massive and in crystalline aggregates. 

The composition expressed by the formula 
given above is Si02=38.i8; CuO= 50.40; 
H20= 11.44, which is approached very closely 
by some analyses. The same composition may 
be expressed by CuSiOa'JfeO. Indeed, recent 
work indicates that the mineral is a hydrated 
metasilicate and not an acid orthosilicate. Fig. 190.— Dioptase Crys- 

Dioptase has an emerald-green or blackish **! with * P2 » x «° and 

green color, a glassy luster and a green streak. ~~ ' ° 221 ^'' "^ a 
Ta . A A , , . - Rhombohedron o£ the 

It is transparent or translucent, is brittle 3rd Qrder Indicated by 

and its fracture is uneven or conchoidal. Its striations. 

hardness is 5 and its density 3.05. It is weakly 

pleochroic and is distinctly pyroelectric. For yellow light, «= 1.6580, 

€=1.7079. 

Before the blowpipe dioptase turns black and colors the flame green. 
On charcoal it turns black in the oxidizing flame and red in the reducing 
flame without fusing. It is decomposed by acids with the production of 
gelatinous silica. 

Synthesis. — Crystals of dioptase have been made by allowing mix- 
tures of copper nitrate and potassium silicate to diffuse through a sheet 
of parchment paper. 

Occurrence and Localities. — The mineral occurs in druses on quartz 
in clefts in limestone, and in gold-bearing placers in the Altyn-Tiibe Mt. 
near the Altyn Ssu River, in Siberia; in crystals on wulfenite and cala- 
mine and embedded in clay near R6zbanya, Hungary; with quartz and 
chrysocolla in the Mindonli Mine, French Congo; in copper mines at 
Capiapo, Chile; and in Peru; at the Bon Ton Mines, Graham Co., 
Ariz.; and near Riverside, Pinal Co., in the same State. In the Bon 
Ton Mines it covers the walls of cavities in the ore, which consists of a 
mixture of limonite and copper oxides. 



348 



DESCRIPTIVE MINERALOGY 



MICA GROUP 

The mica group comprises a series of silicates that are characterized 
by such perfect cleavages that extremely thin lamellae may be split 
from them with surfaces that are perfectly smooth. The lamellae are 
elastic and in this respect the members of the group are different from 
other minerals that possess an almost equally perfect cleavage. Some 
of the micas are of great economic importance, but most of them have 
found little use in the arts. 

The micas may be divided into four subgroups, (i) the magnesium- 
iron micas, (2) the calcium micas, (3) the lithium-iron micas, and (4) 
the alkali micas. Of the latter there are three subdivisions, (a) the 
lithia micas, (b) the potash micas, and (c) the soda micas. 

All the micas crystallize in the monoclinic system (monoclinic pris- 
matic class), in crystals with an orthorhombic or hexagonal habit. 

In composition the micas are complex. The alkali micas are ap- 
parently acid orthosilicates of aluminium and an alkali — the potash 
mica being KH2Al3(Si04)3. Other alkali micas are more acid, and 
some of the magnesium-iron micas are very complex. The members 
with the best established compositions are apparently salts of orthosilicic 

acid, and, hence, the entire group is placed 
with the orthosilicates. 

All the micas possess also, in addition to 
the very noticeable cleavage which yields 
the characteristically thin lamellae that are 
so well known, other planes of parting 
which are well exhibited by the rays of 
the percussion figure (Fig. 191). The 
largest ray — known as the characteristic 
ray — is always parallel to the clinopinacoid. 
In some micas the plane of the optical 
axes is the clinopinacoid and in others is 
perpendicular thereto. In the latter, known 
as micas of the first order, the plane of 
the axes is perpendicular to the characteristic ray and in the former, 
distinguished as micas of the second order, it is parallel to this ray. 

The value of the optical angle varies widely. In the magnesia micas 
it is between o° and 15 , in the calcium micas between roo° and 120 , 
and in the other micas between 55 and 75 . When the angle becomes 
zero the mineral is apparently uniaxial. But etch figures on all micas 
indicate a monoclinic symmetry (compare Fig. 194). 



>* 





Fig. 191. — Percussion Figure 
on Basal Plane of Mica. 
The long ray is parallel to 
00 P ob (010). 



ANHYDROUS ORTHOSILICATES 



349 



THE MAGNESIUM-IRON MICAS 

Biotite ((K-H) 2 (Mg-Fe)2(Al-Fe)2(Si0 4 ) 3 ) 

The magnesium-iron micas are usually designated as biotite. This 
group includes micas of both orders as follows: 



ist Order 
Anomite 



2d Order 
Meroxene 
Lepidomelane 
Phlogopiie 




The crystals of biotite have an axial ratio .5774 : 1 : 3.2904 with 
0= 90 . They are usually simple combinations of oP(ooi), 00 P 00 (010), 
— JP(ii2) and P(Tn) (Fig. 192). Twins are 
common, with the twinning plane perpendic- 
ular to oP(ooi). The composition face may 
be the same as the twinning plane or it may be 
oP(ooi) (Fig. 193). The crystals have an 
hexagonal habit, the angle In A 010 being 
6o° 2 2 J'. The mineral also occurs in flat 
scales and in scaly aggregates. 

The color of biotites varies from yellow, 
through green and brown to black. Pleochroism is strong in sections 
perpendicular to the perfect cleavage, i.e., perpendicular to oP(ooi). 
The streak of all varieties is white. Their hardness =2.5 and density 
2 -7~3« I > depending upon composition. The refractive indices for yellow 



Fig. 192. — Biotite 
with oP, 001 (c)\ 
010 (b); P, In 
-IP, 112 (*). 



Crystal 
00P00, 

(ju) and 










6 


\ A \ 


£ 


I - ) 



Fig. 193. — Biotite Twinned about a Plane Perpendicular to oP (001), and Parallel 
to the Edge Between oP(ooi) and — 2P(22i). The composition plane is 
oP(ooi). Mica law. A = right hand twin, B and C = left hand twins. 

light in a light brown biotite from Vesuvius are: a=i.54i2, 0= 1.5745. 
They are higher in darker varieties. 

Etch figures are produced by the action of hot concentrated sulphuric 
acid. 

Varieties and their Localities. — Anomite is rare. It occurs at Green- 
wood Furnace, Orange Co., N. Y., and at Lake Baikal, in Siberia. 



350 DESCRIPTIVE MINERALOGY 

Merozene is the name given to the common biotite of the 2d order. 
It occurs in particularly fine crystals in the limestone blocks included 
in the lava of Mte. Somma, Naples, Italy; at various points in Switzer- 
land, Austria and Hungary; and at many other points abroad and in 
this country. 

Lepidomelane is a black meroxene characterized by the presence 
in it of large quantities of ferric iron. It is essentially a magnesium-free 
biotite. It occurs in igneous rocks, especially those rich in alkalies. 
Two of its best known occurrences in the United States are in the nephe- 
line syenite at Litchfield, Maine, and in a pegmatite in the northern part 
of Baltimore, Md. 

Phlogopite, or amber mica, is the nearly pure magnesium biotite 
which by most mineralogists is regarded as a distinct mineral, partly 
because in nearly all cases it contains fluorine. Its color is yellowish 
brown, brownish red, brownish yellow, green or white. Its luster is 
often pearly, and it frequently exhibits asterism in consequence of the 
presence of inclusions of acicular crystals of rutile or tourmaline arranged 
along the rays of the pressure figure. Its axial angle is small, increasing 
with increase of iron. Its refractive indices are: a= 1.562, 0= 1.606, 
7=1.606. 

Phlogopite is especially characteristic of metamorphosed limestones. 
It occurs abundantly in the metamorphosed limestones around Easton, 
Pa.; at Edwards, St. Lawrence Co., N. Y., and at South Burgess, 
Ontario, Canada. It is also found as a pyrogenetic mineral in certain 
basic igneous rocks. 

Typical analyses of the four varieties of biotite follow: 

I II in IV 

Si02 40.81 35.79 32.35 39.66 

Ti0 2 3-51 tr. .56 

AI2O3 16.47 x 3-7° *7-47 17.00 

Fe2C>3 2.16 4.04 24.22 .27 

FeO 5.92 1709 13. n .20 

MnO .40 1.20 

CaO 1.48 

BaO .33 .62 

MgO 21.08 9.68 .89 26.49 

Na20 1.55 -45 7-°° -6° 



ANHYDROUS ORTHOSILICATES 351 



I II III IV 

K2O 9.01 8.20 6.40 9.97 

H 2 0— \ .90 \ .66 

H 2 0+ ) 2I9 3^6 I 4 " 67 2 . 3 3 

F .10 2 . 24 

Total 

(lessO=F) 99.19 99 .91 100.83 99.66 

I. Anomite from Greenwood Furnace, Orange Co., N. Y. 
II. Meroxene from granite, Butte, Mont. 

III. Lepidomelane from eleolite syenite, Litchfield, Maine. 

IV. Brown phlogopite from Burgess, Can. 

Before the blowpipe the dark, ferruginous varieties fuse easily to a 
black glass; the lighter colored varieties with greater difficulty to a 
yellow glass. Their powder reactions are strongly alkaline. The 
minerals are not attacked by HC1 but are decomposed by strong 
H2SO4. In the closed tube all varieties give a little water. 

The biotites are distinguished from all other minerals except the other 
micas by perfect cleavage and from other micas by their color, solubility 
in strong sulphuric acid and pleochroism. 

The commoner alteration products of biotite are a hydrated biotite, 
chlorite (p. 428), epidote, sillimanite and magnetite, if the mica is 
ferriferous. At the same time there is often a separation of quartz. 
Phlogopite alters to a hydrophlogopite and to penninite (p. 429), and 
talc (p. 401). 

Syntheses. — The biotites are common products of smelting operations. 
They have been made by fusing silicates of the proper composition with 
sodium and magnesium fluorides. 

Occurrences and Origin. — The biotites are common constituents of 
igneous and metamorphic rocks and pegmatite dikes. They also are 
common alteration products of certain silicates, such as hornblende 
and augite. They are present in sedimentary rocks principally as the 
products of weathering. 

Uses. — Phlogopite is used as an insulator in electrical appliances 
and to a less extent for the same purposes as those for which ground 
muscovite is employed. No " amber mica " is produced in the 
United States. Most of that used in this country is imported from 
Canada. 



352 DESCRIPTIVE MINERALOGY 

THE CALCIUM MICAS 

Margarite (Ca(A10) 2 (A10H) 2 (Si0 4 )2) 

Margarite, the calcium mica, is like biotite in the habit of its crys- 
tals, which, however, are not so well formed as these. Usually the min- 
eral occurs in tabular plates with hexagonal outlines but without side 
planes. It occurs also as scaly aggregates. 

Analyses of specimens from Gainsville, Ga. (I), and Peekskill, N. Y. 
(II), gave: 



Si0 2 


AI2O3 


FeO 


MgO 


CaO 


Na 2 


H 2 


Total 


I. 31.72 


50 03 


• • m m 


.12 


"•57 


2.26 


4.88 


100.58 


II- 3 2 -73 


46.58 


512 


1. 00 


11.04 


• • • • 


449 


100.96 



The mineral has a pearly luster on its basal planes, and a glassy luster 
on other planes. Its color is white, yellowish, or gray and its streak 
white. It is transparent or translucent. Its cleavage is not as perfect 
as in the other micas, and its cleavage plates are less elastic. Its hard- 
ness varies from 3 to over 4 and its density is 3. It is a mica of the 
first order. 

Before the blowpipe it swells, but fuses with great difficulty. It 
gives water in the closed tube and is attacked by acids. 

Occurrence. — Margarite is associated with corundum. It is also 
present in some chlorite schists. In all cases it is of metamorphic origin. 

Localities. — It occurs in the Zillerthal, Tyrol; at Campo Longo, in 
Switzerland; at the emery localities in the Grecian Archipelago; at 
the emery mines near 'Chester, Mass.; in schist inclusions in mica 
diorite at Peekskill, N. Y.; with corundum at Village Green, Penn.; 
at the Cullakenee Mine, in Clay Co., N. C; and at corundum local- 
ities in Georgia, Alabama and Virginia. 

THE LITHIUM-IRON MICAS 

Zinnwaldite ((Li- K Na) 3 FeAl(Al(F- OH)) 2 Si 5 Oi 6 ) 

The principal lithium-iron mica, zinnwaldite, is a very complex 
mixture that occurs in several forms so well characterized that they have 
received different names. All of them contain lithium, iron and fluorine, 
but in such different proportions that it has not been possible to ascribe 
to them any one generally acceptable formula. Some of the most im- 
portant of these varieties have compositions corresponding to the fol- 
lowing analyses: 



ANHYDROUS ORTHOSILICATES 353 

i n m iv 

Si02 4019 59- 2 5 5*46 45-87 

AI2O3 22.79 12.57 16.22 22.50 

Fe203 I 9-78 2.21 .66 

FeO .93 7.66 11. 61 

MnO 2.02 .06 1.75 

Na20 7 . 63 .95 .42 

K 2 7.49 5 37 IO - 6 5 10.46 

Li20 3.06 904 4-83 3.28 

F 3-99 7-32 744 7-94 



Total.. 99.32 102. 11 102.71 105.48 



—0=F=.. 9764 9905 99.60 102.15 

I. Rabenglimmer from Altenberg, Saxony. Greenish black with greenish gray 

streak. Sp. gr. =3.146-3.190. 
II. Polylithionite from Kangerluarsuk, Greenland. White or light green plates. 
Sp. gr. = 2.81. 

III. Cryophyllite from Rockport, Mass. Strongly pleochroic green and brown- 

ish red crystals. Sp. gr. = 2.000. Contains also .17 MgO and 1.06 HjO. 

IV. Zinnwaldite from Zinnwald, Bohemia. Plates, white, yellow or greenish 
gray. Sp. gr. = 2.956-2.987. Contains also .91 H 2 and .08 PiO* 

Zinnwaldite occurs in crystals with an axial ratio very near that 
of biotite, and a tabular habit. Twins are like those of biotite with 
ooP(no) the twinning plane. 

It has a pearly luster, is of many colors, particularly violet, gray, 
yellow, brown and dark green and is strongly pleochroic. Its streak is 
light, its hardness between 2 and 3 and its density between 2.8 and 3.2. 
It is a mica of the second order. 

Before the blowpipe it fuses to a dark, weakly magnetic bead. It is 
attacked by acids. 

Occurrence and Localities. — Zinnwaldite is found in certain ore veins, 
in granites containing cassiterite, and in pegmatites. Its origin is as- 
cribed to pneumatolytic processes. Its principal occurrences are those 
referred to in connection with the analyses. 

THE ALKALI MICAS 

The alkali micas include those in which the principal metallic con- 
stituents besides aluminium are lithium, potassium and sodium. All 
these metals are present in each of the recognized varieties of the 
alkali micas, but in each variety one of them predominates. That in 
which lithium is prominent is known as lepidolite; that in which potas- 



354 DESCRIPTIVE MINERALOGY 

sium is most abundant is muscovite; and that in which sodium is 
most prominent is paragoniie. Muscovite is common. Lepidolite is 
abundant in a few places. Paragonite is rare. The first two are im- 
portant economically. All are micas of the first order, except a few 
lepidolites, and all are light colored. 

Another mica, which is usually regarded as a distinct variety of 
muscovite, or, at any rate, as being very closely related to the mineral 
is roscoelite. In this, about two-thirds of the AI2O3 in muscovite is 
replaced by V2O3. It is a rare green mica which is utilized as an ore 
of vanadium. 

Lepidolite ((Li-K.Na) 2 ((Al-Fe)OH) 2 (Si0 3 )3) 

Lepidolite occurs almost exclusively as aggregates of thin plates 
with hexagonal outlines. Crystals are so poorly developed that a satis- 
factory axial ratio has not been determined. Its variation in composi- 
tion is indicated by the analyses of white and purple varieties from 
American localities. 

in III IV 

Si02 51S 2 49-5 2 51-12 5125 

AI2O3 25 . 96 28 . 80 22 . 70 25 . 62 

Fe2C>3 .31 40 .80 .12 

FeO undet. .24 .00 

MnO .20 .07 i-34* 05 

MgO .02 .02 .00 

CaO 16 .13 tr. 

Li20 4.90 3.87 5.12 . 4.31 

Na20 1.06 .13 2.28 1.94 

K2O 1 1. 01 8.82 10.60 10.65 

Rb 2 3.73 

Cs 2 .08 

F 5.80 5.18 6.38 7.06 

H2O .95 1.72 2.05 1.60 



Total 

(lessO=F) 99.45 100.53 99 74 99 - 6 3 

I. Lilac-purple granular lepidolite from Rumford, Maine. 
II. White variety from Norway, Maine. 

III. Red-purple variety from Tourmaline Queen Mine, Pala, Cal. Contains 

also .04 P2O6. 

IV. White variety from Pala, Cal. 

* Mn 2 O s . 



■■■«• 



ANHYDROUS ORTHOSILICATES 355 

The mineral is white, rose or light purple, gray or greenish. The 
rose and purple varieties contain a little manganese. The streak 
of all lepidolites is white, their luster* pearly, their hardness 2.5-4 
and density 2.8-2.9. The refractive indices of a typical variety are: 
0=1.5975, 7=1.6047. 

Lepidolite fuses easily to a white enamel and at the same time colors 
the flame red. It is difficultly attacked by acids, but after heating is 
easily decomposed. 

Cookeitc from Maine and California is probably a weathered lepido- 
lite. Its analyses correspond to the formula, Li (Al (011)2)3 (Si03)2. 

Occurrence, — The mineral occurs principally in pegmatites in which 
rubellite (p. 435), and other bright-colored tourmalines exist and on 
the borders of granite masses and in rocks adjacent to them. It is 
often zonally intergrown with muscovite. In all cases it is probably a 
pneumatolytic product, or, at least, is produced by the aid of magmatic 
emanations. 

Localities. — The mineral occurs in nearly all districts producing tin, 
and also in those producing gem tourmaline. Its best known foreign 
localities are Jekaterinburg, Russia; Rozna, Moravia; Schnittenhofen, 
Bohemia; and Penig, Saxony. In the United States it is found in large 
quantities at Hebron, Paris, and other points in western Maine; in the 
tin mines of the southern Black Hills, South Dakota, and in the tourma- 
line localities in the neighborhood of Pala, San Diego Co., Cal. 

Uses. — Lepidolite is utilized to a slight extent in the manufacture of 
lithium compounds, which are employed in the preparation of lithia 
waters, medicinal compounds, salts used in photography and in the 
manufacture of fireworks and storage batteries. 

Muscovite (H 2 (K-Na)Al3(Si0 4 )3) 

Muscovite is one of the most common, and at the same time the 
most important, of the micas. Because of its transparency it is em- 
ployed for many purposes for which the darker biotite is not suitable. 

While predominantly a potash mica, nearly all muscovite contains 
some soda, due to the isomorphous mixture of the paragonite molecule. 
Two typical analyses are quoted below: 

SiQi AI2O3 Fe»0, FeO MnO MgO CaO Na«0 K 2 HtO F Total 

I. 44-39 35-70 109 1.07 tr ... .10 2.41 9.77 5.88 .72 101.13 

II. 46.54 34-96 1-59 3 2 ••■ -4i 10.38 5.43 99.63 

I. Broad plates of muscovite bordered by lepidolite, Auburn, Maine. 
II. Greenish muscovite, Auburn, Maine. Total less O = F is 100.83. 



356 



DESCRIPTIVE MINERALOGY 



The crystals are usually tabular and frequently orthorhombic or 
hexagonal in habit, though the etch figures on their basal planes reveal 
clearly their monoclinic symmetry (Fig. 194). If orientated to corre- 
spond with crystals of biotite their axial constants are: a : b : £=.5774 
: 1 : 3.3128, 18=89° 54', and their principal planes oP(ooi), 00 P 00 (010) 
|Pob (023), 4P(44i) and -2P(22i) (Fig. 195). 

Twins like those of biotite are not uncommon in some localities. 

Muscovite is colorless or of some light shade of green, yellow or red. 
It has a glassy luster, a perfect cleavage parallel to the base, a hardness 
of 2 and a density of 2.76-3.1. Pleochroism is marked in directions 
perpendicular and parallel to the cleavage, the color of the crystals, 
when viewed in the direction perpendicular to the cleavage being lighter 





Fig. 194. Fig. 195. 

Fig. 194. — Etch Figures on oP(ooi) of Muscovite, Exhibiting Monoclinic Symmetry. 

Fig. 195. — Muscovite Crystal with — 2P, 221 (M); oP, 001 (c); 00 Poo, 010 (6), 

and §Pob , 023 (e). 



than when viewed parallel to the cleavage. The optical angle is com- 
paratively large (s6°-76°), in this respect being very different from that 
of biotite which is small (2°-22°). The mineral is a nonconductor 
of electricity at ordinary temperatures and a poor conductor of heat. 
Its refractive indices vary somewhat with composition. For yellow light 
intermediate values are as follows: a= 1.5619, 0= 1.5947, 7= 1.6027. 

Before the blowpipe thin flakes of muscovite fuse on their edges to a 
gray mass. In the closed tube the mineral yields water which, in some 
cases, reacts for fluorine. It is insoluble in acids under ordinary con- 
ditions, but is decomposed on fusion with alkaline carbonates. 

Muscovite is very stable under surface conditions. Its principal 
change is into a partially hydrated substance, which may be called 
hydromuscovite. It alters also into scaly chloritic products, into 
steatite (p. 401), and serpentine (p. 398). 



ANHYDROUS ORTHOSILICATES 357 

Damourite is a dense fine-grained aggregate of muscovite, often 
forming pseudomorphs after other minerals. 

Sericite is a yellowish or greenish muscovite that occurs in thin, 
curved plates in some schists. 

Fuchsite is a chromiferous variety of an emerald-green color from 
Schwarzenstein, Tyrol. 

Synthesis. — Crystals of muscovite have been made by fusing anda- 
lusite with potassium fluo-silicate and aluminium fluoride. 

Occurrence. — Muscovite occurs in large, ill-defined crystals in peg- 
matites, and in smaller flakes in granites and other acid igneous rocks, 
in some sandstones and slates and in various schists and other meta- 
morphic rocks. It is found also in veins. It is in some cases an orig- 
inal pyrogenic mineral, in other cases a metamorphic mineral and in 
still other cases a secondary mineral resulting from the alteration of 
alkaline aluminous silicates. 

Localities. — The mineral occurs in all regions where pegmatites and 
acid igneous rocks exist. It is mined in North Carolina, South Dakota, 
New Hampshire, Virginia and other states. While phlogopite (amber 
mica) is produced in some countries all the mica produced in this country 
is of the muscovite variety. 

Uses. — Muscovite is used in two forms, (i) as sheet mica, and (2) 
as ground mica. The sheet mica comprises thin cleavage plates cut 
into shapes. It is used in making gas-lamp chimneys, lamp shades, and 
windows in stoves. The greater portion is used as insulators in 
electrical appliances, though for some forms of electrical apparatus the 
amber mica is better. Because of the comparatively high cost of large 
mica plates, small plates are sometimes built up into larger ones. The 
ground mica consists of small crystals and the waste from the manu- 
facture of sheet mica ground very fine. It is used in the manufacture 
of wall paper, heavy lubricants and fancy paints. It is also mixed with 
shellac and melted into desired forms for electrical insulators. 

Production. — The total value of the mica produced in the United 
States during 191 2 was $355,804, divided as follows: 1,887,201 lb. sheet 
mica, valued at $310,254 and 3,512 tons ground mica, valued at $45,550. 
Of this North Carolina produced 454,653 lb. of sheet mica, valued at 
$187,501 and 2,347 tons of scrap mica, valued at $29,798, or a total 
value for both kinds of mica of $217,299. The imports of sheet mica 
during the same year amounted to $502,163, of which 241,124 lb., 
valued at $155,686 was trimmed and the balance untrimmed. The 
imports during 191 2 were valued at $748,973, and the domestic produc- 
tion at $331,896. 



358 DESCRIPTIVE MINERALOGY 

Roscoelite may be regarded as a muscovite in which a large portion 
of the AI2O3 has been replaced by V2O3. A specimen from Lotus, Eldo- 
rado Co., Cal., gave: 

S1O2 TiO, A1A VA FeO MgO K,0 H,0- H£>+ Total 
45.17 .78 n.54 24.01 1.60 1.64 10.37 .40 4.29 99.80 

besides traces of Li20 and Na20. 

The mineral occurs as clove-brown or green scales with a specific 
gravity of 2.92-2.94. It is translucent and has a pearly luster and a 
strong pleochroism. Its refractive indices for sodium light are : a = 1 .6 io, 
0= 1.685, 7=1.704. 

Before the blowpipe it fuses to a black glass. It gives the usual 
reactions for vanadium in the beads and is only slightly affected by 
acids. It has been found associated with gold in small veins near Lotus, 
Eldorado Co., California, in seams composed of roscoelite and quartz 
between the beds of a sandstone in the high plateau region of south- 
western Colorado, and as a cement of minute scales between the grains 
of the sandstone on both sides of the seams. In all cases it appears to 
have been deposited by percolating water, possibly of magmatic origin. 

The impregnated sandstone is mined as a source of vanadium. The 
material, which contains an average of about 3 per cent of metallic 
vanadium is concentrated by chemical processes, and the concentrates 
are manufactured into ferro-vanadium. Most of the vanadium pro- 
duced in the United States is made from this ore. 

Paragonite (H 2 (Na-K)Al3(Si0 4 )3) 

Paragonite, the sodium mica, differs from muscovite mainly in com- 
position. Both contain sodium and potassium but in paragonite the 
sodium molecule is in excess. 

The analysis quoted below is made on a sample from Monte Cam- 
pione, in Switzerland. 

Si0 2 AI2O3 Fe 2 03 Na 2 K 2 H 2 Total 

47.75 4°i° tr. 6.04 1. 12 4.58 99. 59 

It occurs in the same associations as some forms of muscovite but it 
is much less common. It apparently occurs most abundantly in certain 
fine-grained mica schists to which the name paragonite schists has been 
given. It is in all known cases a product of dynamic metamorphism. 



CHAPTER XVn 
THE SILICATES— Continued 

THE ANHYDROUS METASILICATES • 

NORMAL METASILICATES 

Beryl (BeaAfcCSiC^e) 

Beryl is a frequent constituent of coarse-grained granites. It is 
important as a gem material, and is particularly interesting because of 
the many physical investigations that have been made with the aid of its 
crvstals. 

Although the mineral is essentially a beryllium aluminometasilicate, 
it usually contains also a little Fe20a and MgO, in many cases small 
quantities of the alkalies, and in some cases also caesium. Analyses of 
a green beryl from North Carolina, an aquamarine from Stoneham, Me., 
and a light-colored crystal from Hebron are given below. 

SiCfe AI2O3 Fe 2 03 BeO FeO Na 2 LijjO Cs 2 H 2 Total 

14. 11 100.00 

13.61 .22 .83 99-54 

13.73 ••• -7 1 2 - GI IOO -39 

11.36 .38 1. 13 1.60 3.60 2.03 100.30 

I. Theoretical. 
II. Alexander Co., N. C. 

III. Stoneham, Me.; also .06% CaO. 

IV. Hebron, Me. 

The mineral occurs massive without distinct crystal form and also in 
granular and columnar aggregates, but its usual method of occurrence is 
in sharp and, in some cases, very large columnar crystals with a distinct 
hexagonal habit (dihexagonal bipyramidal class), and an axial ratio 
1 : .4989. The forms found on nearly all crystals are 00 P(ioTo), 
ooP 2 (ii2o), oP(oooi), P(ioTi), P 2 (ii22) and 2P 2 (ii2i) (Fig. 196). 
In addition, there are present on many crystals other prismatic forms 
and the pyramids 3PK2131) and 2P(202i). Other crystals are highly 

359 



I. 66.84 


19-05 


■ • • 


II. 66 . 28 


18.60 


■ ■ • 


III. 65.54 


17-75 


.21 


IV. 62.44 


17-74 


.40 



360 



DESCRIPTIVE MINERALOGY 



modified (Fig. 197), the total number of forms that have been identified 
approximating 50. The angle 10T1 AoiTi = 28° 55'. Some crystals 
are very large, measuring 2 to 4 feet in length and 1 ft. in diameter. 

Beryl has a glassy luster. It is transparent or translucent. It is 
colorless or of some light shade of green, red, or blue. Its streak is 
white, hardness 7-8 and density 2.6-2.8. Its cleavage is very imperfect 
but there is frequently a parting parallel to the base. Pleochroism is 
noticeable in green and blue crystals. Its refractive indices for yellow 
light at 20 are: w= 1.5740, €= 1.5690. They become greater with increas- 
ing temperature. 

Before the blowpipe colorless varieties become milky, but others are 



m 





m 



m 



m 



m 



m 



m 




Fig. 196. Fig. 197. 

Fig. 196.— Beryl Crystals with 00 P, 10T0 (m); oP. 0001 (c); 00 P2, 11 20 (a); P, 

10T1 (p) and 2P2, 1121 (s). 

Fig. 197.— Beryl Crystal with tn, c, p and s as in Fig. 196. Also 2P, 2021 Ga) and 

3P|, 2131 (v). 



unchanged except at very high temperatures when sharp edges are fused 
to a porous glass. The mineral is not attacked by acids. 

Beryl is distinguished from apatite, which it much resembles, by its 
greater hardness. 

It alters to mica and kaolin (p. 404). 

Syntheses. — Beryl crystals have been formed by long heating of 
Si02, AI2O3 and BeO in a melt of the molybdate or vanadate of lithium, 
and by precipitating a solution of beryllium and aluminium sulphates 
with sodium silicate and heating the dried precipitate with boric acid 
in a porcelain oven. 

Occurrence, — The mineral occurs as an accessory constituent in peg- 
matites and granites, in crystalline schists, especially mica schists and 



ANHYDROUS METASILICATES 361 

gneisses, in ore veins and sometimes in clay slates and bituminous lime- 
stones. 

Uses. — The transparent varieties' are utilized as gems, under the 
following names: 

Emerald is a deep green variety, the color of which is probably 

due to O2O3, 

Aquamarine, a blue-green variety, 

Golden beryl, a topaz-colored variety, 

Blue beryl, a blue variety, and 

While beryl, a colorless variety. 

Localities. — Crystals of ordinary beryl occur at Striegau, Silesia; in 
the cassiterite veins near Altenberg, in Saxony; in the granite dikes near 
S. Piero, Elba; in the Mourne Mts., at Down, Ireland; at various points 
(especially near Jekaterinburg), in Ural, Russia, and in North America, 
in the mountain counties of North Carolina; at Mt. Antero, Colo.; at 
Peiperville, Perm.; in granite veins at Haddam, Conn., at Acworth, 
N. H., and at Norway, Hebron, and other points in western Maine. 
Much of the beryl of Maine is the variety containing caesium. 

The finest emeralds are found in geodes, and embedded in a clay 
slate at the Muso Mine, Colombia, New Grenada; but fine gem mate- 
rial occurs also at Zabara, near the Red Sea; Habachthal, Tyrol; Glen, 
New South Wales, and in Brazil, Hindustan and Ceylon. The finest 
aquamarines come from Siberia. 

The most important beryl mines in the United States are in pegma- 
tites in Cleveland, Burke and Macon Counties, N. C. Aquamarine, 
golden beryl and the more usual varieties occur at Walker Knob, Burke 
Co., and on Whiterock Mt. in Macon Co., but those at the first-named 
locality are not clear enough to furnish gems. Near Clayton, Ga., a 
pegmatite contains large bluish and yellowish green beryls, some of which 
yield gem material. The finest aquamarine ever found in the United 
States was from Stoneham, Me. Near Shelby, Cleveland Co., and at 
Crabtree Mountain, Mitchell Co., in North Carolina, genuine emeralds 
occur in a pegmatite that cuts basic rocks. Fine emeralds have also 
been mined at Stony Point, N. C, Haddam, Conn., andTopsham, Me. 

Production. — The total yield of emerald from North Carolina during 
191 2 was about 2,969 carats, valued at $12,875 in the rough. The 
average value of the cut stone was $25 per carat, but some especially 
fine gems from the Shelby locality were valued at $200 per carat. There 
were also produced in the United States during this year other varieties 
of beryl, valued at $1,765. 



362 DESCRIPTIVE MINERALOGY 

Leucite (K 2 Al 2 (Si0 3 )4) 

Leucite occurs almost exclusively in what are apparently simple 
isometric crystals, but which are actually polysynthetic twins of a doubly 
refracting substance. At 500 ° and above, leucite substance is isometric. 
It separates from molten magmas as isometric crystals, which, upon 
further cooling, become twinned. The twinning is revealed by striation 
on the crystal faces. The substance is, therefore, dimorphous. 

Theoretically, leucite is a potassium aluminium metasilicate, but 
most natural crystals contain some soda and many contain small quan- 
tities of calcium. The calculated composition of the pure molecule and 
the actual composition of two natural crystals are shown below. 

Si0 2 AI2O3 CaO Na 2 K 2 H 2 Total 

Calculated 5502 23.40 21.58 100.00 

Mt* Vesuvius . . 55.28 24.08 60 20.79 ••• IOO -75 

Mt. Vulture. .. . 54. 94 25.10 1.80 1.23 15.18 2.13 100.38 

The mineral occurs in icositetrahedrons, 202(211), in some cases 
modified by 00 0(i 10) and 00 O 00 (100). Twinning parallel to 00 0(i 10) 
is common, but often the twins are polysynthetic and are recognizable 
only by striations on the crystal faces. The twinning lamellae are 
anisotropic, as shown by their optical properties, but at 500 the twin- 
ning disappears and the crystals become completely isotropic through- 
out. 

Leucite is glassy in luster and colorless, white or light gray in color. 
It is transparent or translucent and has a white streak. Its cleavage is 
imperfect parallel to 00 O(no), and its fracture is conchoidal or uneven. 
It is brittle. Its hardness is 5-6 and density 2.5. Its indices of refrac- 
tion approximate 1.508. 

Before the blowpipe leucite is infusible. It is soluble in HC1 with 
the production of pulverulent silica. Its powder reacts strongly alka- 
line. 

It is distinguished from other minerals by its crystallization, by the 
violet color it imparts to the flame and its reaction toward HC1. It is 
most apt to be confused with analcile (p. 458) and colorless garnet. It 
is distinguished from the latter by its inferior hardness and from the 
former by its infusibility and failure to yield water when heated in the 
glass tube below red heat. Analcite, moreover, fails to give the flame 
test for potash. 

The mineral alters quite readily into analcite and some other zeolite, 
into a mixture of orthoclase and nepheline, or into orthoclase (p. 413) 



ANHYDROUS METASILICATES 



363 



and muscovite, or into orthoclase alone. Its final alteration product is 
kaolin. 

Syntheses. — Its crystals have been obtained by fusing its constituents, 
and also by melting a mixture of SiCte, potassium aluminate and vana- 
date, and by fusing a mixture of Si02 and AI2O3 with an excess of KF. 

Occurrence. — It occurs only in igneous rocks, especially in lavas low in 
silica and high in potash, and in the plutonic rock known as missourite. 
In some old rocks it is represented by its alteration products. In all 
cases it is a primary mineral. 

Localities. — Leucite is an essential constituent of the lavas in the 
Kaiserstuhl, Baden; in Rhenish Prussia; near Wiesenthal, Saxony; 
in the Siebenburger, Bohemia; at Vesuvius, Italy; in the Leucite Hills, 
and other places in Wyoming, and at several places in Montana; at 
Magnet Cove, Ark., and near Hamburg, N. J. 

Uses. — It is suggested that the large masses of leucite rocks in the 
Leucite Hills be used as a source of potash. On the assumption that 
the rocks at this place contain 10 per cent of K2O it is estimated that 
the total quantity of potash in them amounts to about 200,000,000 tons. 

THE AMPHIBOLOIDS 

The amphiboloids embrace a large number of minerals, some of 
which are extremely important as rock components. Economically, 



A'VX"." ■.:::'. y. 
yy.- "-VX- • • .•.*.'.".■■ f • 



mo 



X ;*//<>.,...'...> ■>•:' y;/->y\ilO 



w 



A 



tW.v '• :..■■■ y-"sy 



100 




100 



B 




Fig. 198. — Cross-Sections of Pyroxene (A) and Amphibole (B) Crystals Illustrating 

Differences in Intersections of Cleavages. 



they have little value. Several are used in the arts, but only to a com- 
paratively slight extent. Apparently they crystallize in the orthorhom- 
bic, monoclinic and triclinic systems. 

The amphiboloids are divisible into two groups, the pyroxenes and 
the amphiboles, which differ from one another in the ratio between their 



364 DESCRIPTIVE MINERALOGY 

a and b axes. In the pyroxenes this ratio is nearly i . i, while in the 
amphiboles it is approximately 2:1. The angle between the prismatic 
planes («>P, no) on the former is nearly equal (87 and 93 °), and on 
the latter very unequal (56°-i24°). Since, moreover, in all members of 
both groups there is a distinct cleavage parallel to the unit prism, the 
angles of intersection of the cleavage planes in the pyroxenes and in the 
hornblendes are also different. This difference in prismatic and cleav- 
age angles of the two groups serves readily to distinguish between them 

(Fig. 198). 

The pyroxenes appear to be the more stable at high temperatures 
and the amphiboles under high pressures. Thus pyroxenes are more 
common than the amphiboles in lavas and amphiboles more common 
than pyroxenes in crystalline schists. 

Chemically, the amphiboloids are metasilicates ot Na, Li, Mg, Ca, 
Fe, Mn, Zn and Al, or isomorphous mixtures of these metasilicates with 
one another and with an orthosilicate of the general composition rep- 
resented by (Mg- Fe)((Al- Fe)0) 2 Si04. 

THE PYROXENES 
(R'SiO,, R'AI(SiO,) a and R^R^OsSiOj 



The pyroxenes occur very widely spread as constituents of igneous 
rocks, and in veins that have been filled by igneous processes. Some 
members of the group are also common metamorphic products. Although 
crystallizing in different systems their crystals possess a certain family 
resemblance, expressed best in their horizontal cross-sections, which 
have a nearly orthorhombic symmetry, i.e., they are nearly symmetrical 
about two planes at right angles to one another, passing through the 
a and b axes, which are nearly equal. The most perfect cleavage of all 
the pyroxenes is parallel to ooP(no), and consequently their cleavage 
angles are nearly equal (Fig. 198A). They approximate 92 ° and 88°, 
with the plane of the a and c axes (the plane of symmetry in monoclinic 
forms) bisecting the acute angle. 

The best known members of the series with their axial ratios are 
listed below. In the case of the orthorhombic members it will be noticed 
that the shorter of the lateral axes is made 1. This is done to empha- 
size the correspondence between the orthorhombic, monoclinic and tri- 
clinic forms in their axial ratios. The usual orientation, that which 
regards the longer of the lateral axes as b(=i) gives a : b : ^=.9702 
: 1 : .5710 for bronzite, and .9713 : 1 : 5700 for hypersthene. By 
many authors wollastonite and pectolite are placed in an independent 



ANHYDROUS METASILICATES 



365 



group partly because of the fact that they are much more easily decom- 
posed by acids than are the other pyroxenes, and partly because of 
their very different crystal habits, and different axial ratios. 



Enstatite 
Bronzite 
Hypersthene 



Wollastonite 

Pectoliie 

Diopside 

Sahlite 

Hedenbergite 

Schejferite 

Augite 

Acmite 
Aegirine 

Jadeite 
Spodumenc 



Rhodonite 
Bustamite 
Babingtonite 
Fowlerite 



Orthorhombic (possibly twinned monoclinic). 

MgSi0 8 b:a:c =1.035 : 1 : .587 

(MgFe)Si0 8 = 1.0308 : 1 : .5885 

FeSiOa =1.0295 : 1 : .5868 



c =1.0523 : 1 
= 1.1140 : 1 



Monoclinic (monoclinic prismatic class). 

CaSi0 3 a : b 

HNaCa*(SiO,) 8 

(MgCa)Si0 8 

(MgFe)Ca(SiO,), 

FeCa(SiOs)2 

(Mg.Fe)(CaMn)(SiO,), 
r(Mg.Fe)Ca(SiO,) 2 
\ (Mg.Fe)((Al.Fe)0)2Si0 4 
lNa(Al.Fe)(Si0 8 ), 

NaFe(SiO,)j 

NaFe(SiOa), \ 

(MgFe)((Al.Fe)0) 2 Si04j 

NaAl(SiO,) a 

LiAl(SiOa), 



.9649 & 
.9864 



= 1.0921 : 1 : .5893 
=1.000 : 1 : .583 

=1.0955 : 1 : -59<H 



= 1.0096 
= 1.098 



.6012 
.601 



= 1.103 : 1 : .613 
= 1.1283 : 1 : .6234 



'84° 35' 
=84° 40' 

74° 11' 



74° 10' 



74° 14' 



= 73° 11' 
= 73° 09' 

= 72° 45' 
=69° 33' 



Triclinic (triclinic pinacoidal class). 

MnSiOs a : b : c =1.0729 : 1 : .6213 0=io8° 44' 

(MnCa)SiOa 

(CaFeMn),Fej(Si08)e =1.0807 : 1 : -6237 /3=io8°34' 

(MnFeCaZn)SiO, 



In addition, there are several comparatively rare monoclinic pyrox- 
enes and one triclinic form, that contain zirconium. They occur only 
as components of rocks rich in alkalies. 



ORTHORHOMBIC PYROXENES 



Enstatite MgSi0 3 )—Bronzite— Hypersthene (FeSiQ 3 ) 



The orthorhombic pyroxenes are isomorphous mixtures of MgSi03 
and FeSi03. The pure magnesium and iron molecules are not known in 
nature, though the former has been produced artificially. Nearly all 
members of the group contain both magnesium and iron. When the 
proportion of the iron present is small (5 per cent FeO), the mixture is 
known as enstatite. Mixtures with 5 to 16.8 per cent of FeO (cor- 



366 



DESCRIPTIVE MINERALOGY 



responding to MgO : FeO= 3 : 1), are known as bronzite and mixtures 
containing more than 16.8 per cent FeO are known as hypersthene. 
The composition of MgSiOs and of some typical members of the group 
follow. 



Si0 2 


AI2O3 


FeO 


MgO 


I. 60.03 


• ■ • a 




39-97 


II. 58.00 


i-35 


3.16 


3691 


HI. 55 50 


■ • ■ ■ 


16.80 


27.70 


IV. 52.12 


1.69 


20.94 


21.56 



CaO H2O 



.80 



320 



Total 
100.00 
100.22 
100.00 

9951 




I. Calculated composition of MgSiOs. 
II. Portion of large crystals of enstatite from Kjorrestad, Norway. 

III. Calculated composition of upper limit of bronzite, i.e., in which MgO : FeO 

=3 : 1. 

IV. Hypersthene powder separated from a gabbro at Mt. Hope, Md. 

The three minerals occur in crystals that have a well marked ortho- 
rhombic symmetry, but it is believed that this may be a case of pseu- 
dosymmetry only, i.e., that the minerals may in reality be monoclinic, 
and that their apparently orthorhombic symmetry may be due to 
repeated polysynthetic twinning of very thin lamellae. Monoclinic 
MgSi03 has been made by fusion of SK>2 and MgO in the presence of 

B2O3, but it is not certain that this is identi- 
cal with an iron-free enstatite. 

The natural crystals of the orthorhombic 

pyroxenes are columnar in habit and are 

usually bounded by ooP(no), 00 Poo (010), 

00 Poo (100), P2(2i2), JP 06 (014), with the 

addition on some crystals of ooP2(i2o), 

fP 86(034), P(lll), 2P2(2Il), £Po6(oi2) 

and other forms (Fig. 199). All cleave per- 
fectly parallel to ooP(no) with a cleavage 
angle of 88° i6'-2o' and 91 ° 4o'-44'. The 
angle noAiio=88° 16' to 88° 20'. 

The color and other physical properties of 
the orthorhombic pyroxenes vary with the 
amount of iron present. Enstatite is light gray, yellow or green. 
Hypersthene is black, dark purple or dark green. Bronzite is brown, 
or some shade lighter than hypersthene and darker than enstatite. 
All colored varieties are pleochroic, the difference in color in different 
directions increasing with the increase in iron. Green, red, yellow and 
brown tints are most prominent. All varieties have a colorless streak. 



m 



m 



Fig. 109. — Enstatite Crys- 
tal with 00 P, no (m); 
00 P 00 , 100 (a); 00 P 00 , 
010 (6); §P«, 023 (q); 
§PX, 012 (A); JP», 
016 (<t>) and JP, 223 (t). 



ANHYDROUS METASILICATES 367 

Many hypersthenes and bronzites exhibit a metallic shimmer on 
oo P 06(010), due to tiny inclusions with their flat sides parallel to 
this direction. The hardness of the orthorhombic pyroxenes varies 
between 5 and 6 and their density between 3.1 and 3.5 increasing with 
the iron present. Their refractive indices for yellow light are: 

Enstatite a= 1.665 0=1.669 7=1.674 

Hypersthene =1.692 =1.702 =1.705 

Before the blowpipe the iron-free members of the series are infusible. 
With increase in iron they become more easily fusible, very ferruginous 
hypersthene melting easily to a greenish black weakly magnetic glass. 
When treated with hydrochloric acid the members near enstatite are 
unattacked, while those near hypersthene are slightly decomposed. 

Syntheses. — Crystals of these pyroxenes have been made by fusing 
the proper components with B2O3, and by heating mixtures of Si(>2 and 
MgCfe. They are frequent constituents of slags. 

Occurrence. — The rhombic pyroxenes occur in igneous rocks, in crys- 
talline schists, in metamorphosed dolomites and in veins that have been 
filled by igneous magmas. They are not very stable under the condi- 
tions at the earth's surface. They weather to serpentine, hornblende 
and rarely to talc. Enstatite occurs also in meteorites. 

Localities. — Good crystals of the orthorhombic pyroxenes are found 
in the volcanic bombs (inclusions in lava) of the Lake Laach district, 
Prussia; in ore. veins at Bodenmais, Bavaria; at Malnas, Hungary; 
in the trachyte of Mont Dore*, France; in apatite veins at Snarum, 
Norway; and in a glassy andesite on Peel Island, Japan. In the United 
States they occur in basic coarse-grained igneous rocks in North Carolina, 
Maryland, and the Highlands of New York and New Jersey, in volcanic 
rocks in Colorado, and at the Corundum Mines, in Georgia. Espe- 
cially fine bi;onzite occurs on Paul's Island, Labrador. 

MONOCLINIC PYROXENES 

The monoclinic pyroxenes comprise a series of isomorphous mixtures 
of monoclinic metasilicates of Na, Li, Ca, Mg, Fe" and Mn and the 
silicate R" (R'"0) 2 Si0 4 , in which R" is usually Mg, Ca or Fe and R'" 
is Al or Fe. 

-Although their chemical composition varies quite widely, the crys- 
tallization of all the members of the group is practically the same. With 
the exception of wollastonite and pectolite the habit of their crystals is 
similar and their corresponding interfacial angles have approximately 
the same value. 



368 



DESCRIPTIVE MINERALOGY 



The group may be subdivided into four subgroups (i) the wollas- 
tonite subgroup, including this mineral and pectolite, with calcium as 
the principal metallic component, (2) the magnesium-calcium-iron 
pyroxenes, including diopslde, sa\lUe y he lender gite and augite, and (3) 
the alkali pyroxenes including acm'U^jaleite and spolumene. A fourth 
subgroup includes the rare zirconium-bearing pyroxenes. All crystal- 
lize in the monoclinic prismatic class. 

Wollastonite Subgroup 

These minerals, because their axial ratios are somewhat different 
from those of the other monoclinic pyroxenes, and because they are 
much more easily decomposed by acids, are by some mineralogists re- 
garded as constituting an independent group. 

Wollastonite (CaSi0 3 ) 

Wollastonite analyses correspond very closely to the theoretical 
composition required by the formula assigned to it. There is, however, 
nearly always a little Fe2C>3 present and usually there are present also 
small traces of other constituents. A dimorph, pseudowollastonite, or 
/3 wollastonite, has been made by melting wollastonite and cooling 
slowly, but it has not yet been found in nature. Its crystals are hexag- 
agonal or monoclinic with an hexagonal habit. 

Si0 2 FeOMnOCaO MgONa 2 H 2 Total 

Theoretical 51 . 75 ... 48.25 100.00 

Bonaparte Lake, N. Y. 50 . 66 .07 47 . 98 .05 .46 .72 99 . 94 

The mineral forms tabular or columnar crystals bounded by 

00 Pob (100), — P 6b(lOl), OP(OOI), Poo(iol), 00P2(l2o), — P2(l22) 

and 00PK540) (Fig. 200). Twins are 
sometimes found with 00 P 60 (100) the 
twinning plane. The angle 540 A 540 = 79 
58'. The mineral occurs also in granular 
and fibrous masses. Its cleavage is per- 
fect parallel to 00 P do (100) and only a 
little less perfect parallel to oP(ooi). 

Wollastonite is usually colorless or 
white, but in some cases is grayish, yellow- 
ish, reddish or brown. It is transparent 
or translucent and has a white streak. Its 
luster is glassy except on the cleavage face 
where it is often pearly. Its hardness is 




7 



-a 



Fig. 200. — Wollastonite Crys- 
tal with oP, 001 (r); 00 P ^ r 
100 (a); -Pm, ioi (»); 
j-Pco, 101 (0; +JP«, 
T02 (a) and ooPf, 54© W. 



ANHYDROUS METASILICATES 369 

4.5-5 and density 2.8-2.9, an d i ts refractive indices for yellow light 
are: <*= 1. 62 1, /3= 1.633, 7=1.636. 

Before the blowpipe wollastonite fuses with difficulty to a white 
transparent glass. Its fusing point varies between 1240 and 1325 , 
diminishing with increase in iron. It dissolves in HC1, leaving a residue 
of gelatinous silica > and is attacked vigorously by strong solutions of 
NaOH. When fused it recrystallizes in hexagonal crystals (pseudo- 
wollastonite). 

The mineral is distinguished from other white silicates by its crys- 
tallization, its cleavage and its solubility in hydrochloric acid. Its prin- 
cipal alteration is into apophyllite (p. 443). 

Syntheses. — Crystals of wollastonite have been made by fusing Si02 
and CaF2, and by dissolving the hexagonal modification (made by fusing 
and cooling wollastonite) in molten calcium vanadate at 8oo°-9oo°. 

Occurrence. — Wollastonite is characteristically a product of meta- 
morphic processes, both contact and regional. It occurs in metamor- 
phosed dolomites, in the limestone inclusions in the lava of Vesuvius, 
etc., in many gneisses and in some eruptive rocks. It is found also 
abundantly in calcareous slags. 

Localities. — Crystals of wollastonite are found in the phonolite of the 
Kaiserstuhl, near Freiburg, Bavaria; in a contact metamorphosed lime- 
stone near Cziklova, Hungary; in the limestone bombs in the lava of 
Mt. Somma, Naples, Italy, and of Santorin, Greece; and in limestone 
at Diana, N. Y. Granular or fibrous masses occur also at Attleboro, 
Penn., at different points in Lewis, Essex and Warren Counties, N. 
Y.; and at the Cliff Mine, Keweenaw Pt., Mich. 

Pectolite (HNaCa 2 (Si0 3 ) 3 ) 

Pectolite was formerly regarded as a partially weathered wollastonite. 
Recent analyses, however, indicate that it may have a definite compo- 
sition which can be represented by the formula written above, as shown 
by the analyses quoted below. The excess of water shown by most 
analyses is ascribed to the admixture of some weathered material. 



Si0 2 AI2O3 


MgO 


CaO 


Na 2 K 2 


H 2 


Total 


I- 54.23 


• • • a 


3372 


9-34 .. 


2.71 


100.00 


II. 4S-3 2 


• • • • 


3400 


9.32 


2-55 


100.30 


III. 53.94 .71 


i-43 


32.21 


8-57 -47 


4.09 


100.82 


I. Theoretical. 












IT. Niakornat, Greenland. Contains also 


.11 per cent FejOs. 






III. Point Barrow, Alaska. 











370 DESCRIPTIVE MINERALOGY 

The mineral usually occurs in fibrous masses of acicular crystals 
elongated in the direction of the orthoaxis, but in a few cases in tabular 
forms flattened parallel to oo P 66 (ioo). Its cleavage is distinct parallel 
to the same plane. 

Pectolite when pure, or nearly pure, is colorless or white or gray, and 
transparent or translucent. Its luster is pearly on cleavage surfaces 
and satiny on fracture surfaces. Its hardness is about 4.5 and its den- 
sity 2.88. When broken in the dark, some specimens phosphoresce. 
Its average refractive index for yellow light is 1.61. 

Before the blowpipe the mineral fuses to a white enamel. It yields 
water when heated in the closed tube and when treated with hot hydro- 
chloric acid it decomposes, leaving a residue of flocculent silica. 

The principal alteration product of pectolite is talc (p. 401). 

Synthesis. — Small, fine needles of pectolite have been produced by 
heating to 400 mixtures of S1O2, AI2Q3, Na20, CaO and H2O, in various 
proportions. 

Occurrence. — The mineral occurs in druses and as isolated crystals on 
the walls of cracks in eruptive rocks, and also in a few instances as vein 
fillings, and as a constituent of metamorphic rocks. It is mainly a 
secondary mineral. 

Localities. — Crystals are found in seams in basalts at Edinburghshire, 
Scotland; at Bergen Hill, N. J., in clefts in traprock; and in the eleolite- 
syenite at Magnet Cove, Ark. {manganopectolite with about 4 per cent 
MnO). At Barrow Point, Alaska, fine-grained fibrous aggregates are 
found in abandoned workshops of the Eskimo. Radially fibrous masses 
occur in the Thunder Bay region, Lake Superior, at Disco, Greenland, 
and at a number of points in the Alps. 

Magnesium-Caicium-Iron Pyroxenes 
Diopside-Augite 

The calcium-magnesium-iron pyroxenes include a number of com- 
pounds that have been given distinctive names. They are apparently 
isomorphous mixtures of the metasilicates of Mg, Ca, Fe and Mn, or of 
these together with the magnesium and iron salts of the basic orthosilicate 
of iron and aluminium (Mg- Fe)((Al* Fe)0)2Si04. 

The crystals of all members of the group are alike in habit and similar 
in their interfacial angles. Their axial ratios are nearly the same and 
the angle has nearly the same value in all. It is possible that the 
slight differences observed are due to the effect of the varying amounts of 
iron present. The crystals are nearly all short columnar in habit, with 



ANHYDROUS METASILICATES 



371 



the vertical zone well developed. The simplest crystals are bounded by 
oo P 60(100), ooP(no), 00 Poo (010) and P(Tn), but — P(in), 
2P(22i), oP(ooi) and 2Pob(o2i) are also common (Fig. 201). Other 
forms to the number of 95 have been observed, but they are compara- 
tively rare. Contact and interpenetration twins are fairly common. 
In the contact twins the usual twinning plane is 00 P do (100) (Fig. 202). 
Polysynthetic twins are twinned parallel to oP(ooi). In the interpene- 
tration twins —P 00(101) (Fig. 203) and P2(l22) are the twinning 
planes. The cleavage is parallel to 00 P(no), the cleavage angles being 
about 93 and 87 °. Partings are also common, parallel to one or the 
other of the three pinacoids. 

All the pyroxenes of this group have a glassy luster and are trans- 
parent or translucent, Their color varies with composition as does also 




m 




m 



m 




m 





Fig. 201. 



Fig. 202. 



Fig. 203. 

Fig. 201. — Augite Crystal with *> P, up (m); 00 P 55 , 100 (a); 00 P So , 010 (b) and 

P, In (5). 

Fig. 202. — Augite Twinned about 00 P 55 (100). 

Fig. 203. — Interpenetration Twin of Augite, with — P55 (101) the Twinning Plane. 

their hardness and density. The limits of hardness are 5 and 6 and of 
density 3.2 and 3.6. The streak of all varieties is white. Pleochroism 
has been observed in some occurrences but it is not as noticeable as in 
the corresponding amphiboles. In the pyroxenes of this group it is 
usually in shades of green, but in the diallage of the Lake Superior region 
it is fairly strong in shades of amethyst. 

Before the blowpipe the members of the group are fusible, their 
fusibility increasing with the quantity of iron present. The fusing 
temperature of the pure diopside is 1381 and of hedenbergite 1100 - 
1 160 . The fusing points of the other pyroxenes of the group lie between 
these temperatures. None of the varieties are attacked by acids to any 
appreciable degree. 

All the pyroxenes are distinguished from other minerals by their 
crystallization and their cleavage. 



372 



DESCRIPTIVE MINERALOGY 



Diopside is a mixture of the magnesiumand calcium silicates in which 
the two molecules are in the ratio i : i. With the addition of the cor- 
responding iron molecule diopside grades into sahlite. The calculated 
composition of a mixture of the formula MgCa(SiOs)2 is indicated in 
the first line. The compositions of several typical diopsides are quoted 
in the following two lines. 



Theoretical 

Albrechtsberg, Aus. . . 
Alathal, Switzerland. 



Si0 2 AI2O3 Fe 2 3 FeO MgO CaO Total 

55 55 18.52 25.93 100.00 

55.60 .16 ... .5618.34 26.77 101.43 

54.28 .51 .98 1. 91 17.30 25.04 100.02 





m 




Fig. 204. — Diopside Crystals with » P, no 
(m) ; ooPm, 100 (a); 00 P « , 010 (b) ; oP, 
001 (c), -P, in («); +2P, 221 (0); 3P3,' 
311 (A); +P06, Toi (J>). 



Its crystals are usually characterized by the presence of the basal 

plane (Fig. 204). The value 
of the angle iioAiTo=92° 

So'- 

Diopside is usually light 

green or colorless, yellowish, 
dark green or nearly black 
and rarely deep blue. The 
lighter varieties are transpar- 
ent or translucent, the darker 
ones opaque; The density of 
the pure mineral is 3.25. Its 
refractive indices for yellow 
lightare:a= 1.6685,0= 1.6755, 
7=1.6980. All these values 

increase with increase in the iron molecule. Among the varieties that 

have been given distinct names may be mentioned: 
Malacolite, a pale colored translucent variety, and 
Chrome-diopside, a bright green variety containing from one to 

several per cent CfoOa. 

Diopside occurs in igneous rocks and in metamorphosed limestones. 

Hedenbergite is the calcium-iron pyroxene, though it always con- 
tains some of the diopside molecule. The calculated compositions of the 
type mineral (FeCaS20e) and of a specimen from its best known locality 
are: 

Si0 2 AI2O3 Fe 2 3 FeO MgO CaO Total 

Theoretical 4839 29 .43 22.18 100.00 

Tunaberg, Sweden . . 47.62 1.88 .10 26.29 2.76 21.53 100.18 



ANHYDROUS METASILICATES 



373 



The mineral is black, except varieties that contain Mn which are 
grayish green. It occurs in crystals (Fig. 205) 
and in lamellar masses. Its density is 3.31, and 
refractive indices for yellow light, a =1.73 20, 
18=1.7366, 7=1.7506. 




Sahlite. — Intermediate between diopside and 

hedenbergite are several pyroxenes which are *j jlj 

characterized by possessing all three of the \ j»\s, 

elements Ca, Mg and Fe in notable amounts. ^ 2^^ 

Of these the most common is sahlite, which is Fig. 205.— Hedenbergite 

grayish, grayish green or black. It occurs in Crystal. Forms a t m, 

crystals and granular masses. *» c ' °> ?> u and \ M 

a 4. ' 1 1 • c 11 *.i_ • in Fig. 204. Also 2P 00. 

A typical analysis follows, the specimen f x , B - 

• / xt 1 i 1 - t. 1 ° 21 W and °° p 5» 5"> 

coming from Valpeleina, Italy: ( x y 



SiCb 


AI2O3 


FeO 


MgO 


CaO 


Total 


54 02 


.20 


8.07 


13 -52 


24.88 


100.69 



Schefferite is a brown or black pyroxene characterized by the fact 
that it contains considerable manganese. It may be regarded as heden- 
bergite in which a portion of the iron molecule has been replaced by the 
corresponding manganese molecule. A specimen from the best known 
locality for the species — Langban, Sweden — gave: 

Si02= 52.28, FeO=3-83, MnO=8.32, MgO= 15.17, CaO= 19.62 = 99.22. 

It occurs in tabular crystals that are usually elongated in the direction 
of the zone ooPob(oio), P(Tn), P<x> (Toi) and in crystalline masses. 

The mineral is yellowish brown or black, according to the percentage 
of iron present. Its sp. gr. is 3.46-3.55 and its fusing temperature 

I200°-I250°. 

A fine blue variety, known as violan, from St. Marcel, Italy, is char- 
acterized by the presence of about 5 per cent Na20, due possibly to the 
admixture of NaMn(SiOs)2. Its sp. gr.=3.2i. 

Jeffersonite is a variety containing zinc, occurring at Franklin Fur- 
nace, N. J. It is found in large crystals with rounded edges. Its color 
is greenish black on fresh fractures and chocolate brown on exposed sur- 
faces. An analysis yielded: 



Si0 2 


AI2O3 


FeO 


MnO 


ZnO 


MgO 


CaO 


H 2 


Total 


49-01 


i-93 


i°-53 


7.00 


4-39 


8.18 


iS-48 


1.20 


98.62 



374 DESCRIPTIVE MINERALOGY 

Augite is the name given to the Ca-Mg-Fe pyroxenes containing 
alumina. They are isomorphous mixtures of (Ca, Mg, Fe) Si03 with 
the alumino and ferric orthosilicates of the same metals, and often with a 
small quantity of the acmite or jadeite molecuje. The varieties of augite 
are numerous, their composition and properties differing with the pro- 
portions of the various molecules in the compounds. The three most 
prominent varieties are: 

Fassaite, a pale to dark green richly magnesian variety. Sp. gr. = 
2.98. 

Ordinary augite, a dark green or brownish black variety, common 
in igneous rocks. Specific gravity 3.24. For yellow light, a=i.7i2, 
£=1.717,7=1.733. 

Diattage, a variety that is characterized by the possession of a 
distinct parting and a lamellar structure, usually parallel to 00 P 66 
(100). 

Omphacite is a bright green sodic variety. Sp. gr. = 3-33. Analyses 
of fassaite (I), of three varieties of augite (II, III, IV) and of ompha- 
cite (V) follow: 

Si02 AI2O3 Fe 2 03 FeO MgO CaO Na 2 Loss Total 

70 100.10 

100.02 

99.90 

100.32 

4.51 .05 100.15 

I. Grass green, Fassathal, Tyrol. 
II. Yellow, Monte Somma, Italy. 

III. Dark green, Monte Somma, Italy. 

IV. Black, Monte Somma, Italy. 

V. Omphacite from the Eclogite of Otztal, Tyrol. Also .92% K 2 and .46% TiO*. 

The augites are usually in short prismatic crystals (Figs. 201, 202). 
They are common constituents of igneous rocks. 

All the pyroxenes of this group are subject to change under the 
conditions on the earth's surface. Under the influence of the weather 
they alter to chlorite. Under metamorphosing conditions they change 
into the corresponding amphiboles, more particularly into the bright 
green variety known as uralite. Alteration to serpentine is also 
common. Steatite, tremolite, epidote and other minerals are also 
frequent alteration products. 



I. 


41-97 


10.63 


736 


•55 


26.60 


10.29 


II. 


50 -4i 


6.07 


1.09 


6.78 


12.92 


22.75 


III. 


51.01 


4.84 


3.5i 


3.16 


16.58 


20.80 


IV. 


46.95 


9-75 


447 


4.09 


16.04 


19.02 


V. 


5421 


10.91 


3.12 


i-33 


10.03 


14.61 



ANHYDROUS METASILICATES 375 

Syntheses. — Diopside and augite are common in furnace slags. They 
have been made by fusing their constituents in open crucibles, with or 
without the addition of a flux. Molten hornblende crystallizes as 
monoclinic pyroxene. 

Occurrence. — The most common methods of occurrence of the various 
pyroxenes have already been mentioned. The magnesium-calcium 
varieties such as diopside and sahlite are found principally in metamor- 
phic limestones. The green varieties are most common in schists and the 
black varieties in igneous rocks, especially the basic ones. Augite often 
occurs also in ore veins, especially with magnetite. 

Localities. — The occurrences of the various pyroxenes are so numerous 
that they cannot be enumerated here. It will be sufficient to state that 
good crystals of diopside are found in the Ala Valley, Piedmont; at Zer- 
matt, in Switzerland; at Pargas, in Finland; and Nordmark, in Sweden. 
Hedenbergite occurs at Tunaberg, Sweden, and Arendal, Norway; 
schefferite at Langban, Sweden, and augite at Mt. Monzoni, in the 
Fassathal; Traversella, Piedmont; Mt. Vesuvius, Italy; the Sandwich 
Islands and the Azores. 

In the United States good crystals are found at Raymond and Rum- 
ford, Me. (diopside, sahlite) ; at Edenville and Dekalb, N. Y. (diopside) ; 
and at Franklin Furnace, N. J. (hedenbergite and jeffersonite). 

Alkali Pyroxenes 

The alkali pyroxenes are characterized by the presence in them of 
alkalis, especially sodium. They may be regarded as isomorphous mix- 
tures of the sodium, lithium, iron and aluminium metasilicates, thus 
Na 2 Si03+Fe 2 (Si03)3=2NaFe(Si03)2, or Na 2 Si0 3 +Al 2 (Si0 3 )3=2NaAl 
(Si03) 2 . The three most common alkali pyroxenes are acmite f jadeite 
and spodumene. Spodumene is used as a source of lithium. Jadeite 
was formerly a favorite material from which to carve sacred emblems. 

Acmite — Aegirine 

Acmite has a composition corresponding to the formula NaFe(SiC>3) 2 , 
and is rare. More commonly this molecule is mixed with the augite 
molecule in the compound known as aegirine or aegirite, or aegirine- 
augite, according to the proportion of the augite molecule present. 
When the mixture contains about 2.50 per cent Na 2 the correspond- 
ing mineral is usually known as aegerine-augite. When MgO and CaO 
are absent (Na 2 0= 12-13 per cent), it is known as acmite. Between 
these limits it is aegirine. 

The calculated compositions of the pure acmite molecule and the 



376 DESCRIPTIVE MINERALOGY 

composition of specimens of acmite, aegirine and aegirine-augite as 
found by analyses are: 

SiOa AI2O3 FeaOa FeO MgO CaO NaaO K 2 Total 

I. 51.97 .... 34.60 13.43 100.00 

II. 51.66 28.28 5.23 12.46 .43 100.25* 

III. 49.31 4.88 16.28 5.65 4.28 9.30 8.68 .68 100.41 f 

IV. 50.33 .30 12.37 10.98 22. 01 2.14 -94 99-73t 

I. Theoretical acmite. 
II. Acmite, Rundemyr, Norway. 
HI. Aegirine, Sarna, Daleltarlien. 
IV. Aegirine-augite, Laurvik, Norway. 

• Contains also .69 per cent MnO, 39 per cent H,Oand 1.11 per cent TiOi. 
t Contains also 1.15 per cent TiOi. 
t Contains also .66 per cent TiOi. 

Acmite crystals are usually more acicular in habit than those of the 
ordinary pyroxenes, and their terminations are steeper. P(Tn) and 
P^(ioi) are common and 6P(56i) and other 
steep pyramids are not uncommon (Fig. 206). 

The mineral has a vitreous luster, and is 
transparent or translucent. Its color is reddish 
brown to brownish black and in some cases 
green. Its hardness is 6 and sp. gr. = 3-52. Its 
refractive indices for yellow light are: 0=1.7630, 
0=1.7990, 7=1.8126. 

Aegirine is greenish black. Its streak is 
yellowish gray or dark green. Pleochroism is 
Fie. 106.— Acmite Crys- strong in green and brown tints. Hardness is 6 
tal with » P s , 100 an d density 3.52. 

(a), =0 =0 010 ( ), Before the blowpipe acmite and aegirine fuse 

« P, 110 (m); +P, 

In (s)- +iPi in t0 a D ' ac ' i magnetic globule. The fusing tem- 
(5) ; -|-6P : 56i (o) perature of acmite is from 970 to 1020 °. Both 
and 8P, 881 (ft). O minerals are slightly attacked by acid before and 
and G merge. gj ter fusing. 

Synthesis. — Acmite has been made by the 
fusion of a mixture of powdered quartz, Fe203 and Na2C(>3 in the pro- 
portions indicated by the formula NaFe(SiC>3)2. 

Occurrence. — Both minerals are limited in their occurrence to soda- 
rich igneous rocks, in which they are primary. 

Localities. — Crystals of acmite occur in a dike of pegmatite near 
Eker, Norway, and in a nepheline syenite at Ditro, Hungary. 



ANHYDROUS METASILICATES 377 

Aegirine crystals are more common. They occur abundantly in the 
nepheline syenite dikes in the neighborhood of Langesundf jord, Norway, 
in some instances in crystals a foot long. They are found also in can- 
crinite syenites at Elfdalen and elsewhere in Sweden; in nepheline 
syenite on the Kola Peninsula, Russia; and in the same rock at Hot 
Springs, Ark. 

Jadeite (NaAl(Si0 3 ) 2 ) 

Jadeite is not known in measurable crystals, but, because sodium 
is almost universally present in the mineral spodumene, where it is ap- 
parently in isomorphous mixture with LiAl(SiOs)2, it is assumed that the 
molecule NaAl(SiC>3)2 crystallizes in the same way as the spodumene and 
the acmite molecules. Most specimens of jadeite are isomorphous mix- 
tures of the jadeite and diopside molecules. When in addition to these 
there is a notable admixture of the acmite molecule, NaFe(Si03)2, 
the mineral is known as chloromelanite. 

The mineral is of great ethnological interest because so many orna- 
ments were made of a rock composed mainly of jadeite by the ancient 
inhabitants of China, Mexico, South America and elsewhere. " Jade " 
ornaments, however, are not all made of jadeite, but in all instances their 
material resembles this mineral in color, structure and density. Many 
of them are made of fibrous amphiboles, some of which correspond to 
jadeite in composition. 

The theoretical composition of the mineral is given in line I, and the 
analyses of specimens from Mexico and China in lines II and III: 

Si0 2 AI2O3 FeO MgO CaO Na 2 K 2 H 2 Total 

I. 59.39 25.56 

II. 58.18 23.53 l6 7 

III. 58.68 21.56 .94 

I. Theoretical. 
II. Oaxaca, Mexico. 
III. Ornament, China. 

Jadeite occurs in fibrous, flaky and dense, finely granular masses 
with a glassy luster, inclining to pearly on cleavage surfaces. Its color 
is in some cases white or yellowish white, but more frequently bright 
green or bluish green. Its streak is white. Its cleavages make angles 
of 87 °. Its fracture is tough and splintery. Its hardness is 6.7 and its 
density 3.3-3.35. Its intermediate index of refraction, 0= 1.654. 

Before the blowpipe jadeite fuses easily to a transparent, blebby glass. 
It is unattacked by acids. After fusion, however, it is easily decomposed 



• • • • 


• • • » 


15-35 


• • • 


■ • • 


100.00 


1.72 


2-35 


11. 81 


•77 


•53 


100.56 


2.49 


3-37 


13.09 


•49 


• • • 


100.62 



378 DESCRIPTIVE MINERALOGY 

by HC1 and sometimes by Na2C(>3. At high temperatures (225 - 
23 5 ) it is also decomposed by water. 

Jadeite alters by metamorphic processes to a white hornblende 
(tremolite). 

Localities. — Ornaments and instruments made of jadeite, and water- 
worn fragments of the mineral are known from many localities in China, 
Tibet, Burma, Switzerland, France, Egypt, Italy, Mexico and Central 
America. The original sources of the material of the ornaments are 
not known. The mineral, however, occurs with albite and nepheline 
in a dike at Tawman, Burma, and probably as a constituent in some 
metamorphic schists. 

Spodumene (LiAl(Si0 3 )2) 

Spodumene is essentially the lithium molecule corresponding to the 
sodium molecule jadeite. Nearly always, however, the mineral contains 
some of the sodium molecule, and a small quantity of helium. Three 
typical analyses are quoted below: 





Theoretical 


Si02 . . . 


64.49 


AI2O3 . . 


27-44 


Fe203. . . 




Mn203. 




FeO. . . 




CaO . . . 




LJ2O . . . 


8.07 


Na 2 . . 




K 2 0... 




H 2 0... 




Total. 


100.00 



Colorless, 


Yellowish green, 


Kunzlte, 


Branchville, 


Minas Geraes, 


S. Diego 


Conn. 


Brazil 


Co., Cul. 


64.25 


64.32 


64.42 


27.20 


27.79 


27.32 


.20 


.67 
•17 


•15 


7.62 


7-45 


7.20 


•39 


•55 


•39 
•03 


.24 


.12 





99.90 101.07 99 5i 

Crystals are usually columnar parallel to 00 P (no) or tabular par- 
allel to 00 P 6b (100) (Fig. 207). They are more complex than those of 
the members of the diopside-augite group and their habit is different. 
The most frequent forms are 00 Poo (100), 00 Poo (010), ooP(iio), 
ooP2(i2o), ooP3(i3o), 2Pob(o2i), 2P(22i) and P(Tn). Some of 
them are of enormous size. In the Etta Mine, Black Hills, South 
Dakota, are many 30 ft. long and 3-4 ft. in diameter. One meas- 
ured 47 ft. in length. Most crystals are striated vertically. Twins are 



ANHYDROUS METASILICATES 



379 




Fig. 207. — Spodumene Crys- 
tal with «P(», 100 (a); 
00 P« # 010 (6); ooP, no 
(w); 00P 2, 120 (/<); 00 P3, 
130 (»); 2P00, 021 (rf); 
+2P, 2_2i_(r); +P, in 
(/>); 2P2, 211 (/) and Poo, 
101 (p). 



fairly common, with ooP(no), the twinning plane. Although crystals 
are not uncommon the mineral more frequently occurs as platy or scaly 
aggregates. The angle 1 10 A iTo= 93 °. 

Spodumene has a glassy luster, which is pearly on cleavage surfaces. 
Its color is white, gray, greenish or yellowish 
green, or amethystine. It is transparent or 
translucent, and its streak is white. Its 
fracture is uneven or conchoidal, its hardness 
between 6 and 7 and its density 3.2. Dark 
green crystals exhibit marked pleochroism. 
Refractive indices for yellow light in speci- 
mens from North Carolina are: 0=1.651, 
/3= 1.669, 7=1.677. 

Two varieties have been named and used 
as gems. These are: 

Hiddenile, a glassy emerald-green variety, 
from Alexander Co., N. C. 

Kunzite, a pink or lilac variety, from 
San Diego Co., California. Under the influ- 
ence of radium rays it becomes green. When 
heated to 240 it becomes a darker rose 
color, but at 400 it loses all color. 

Before the blowpipe the mineral swells up and fuses to a colorless 
glass, at the same time imparting a crimson color to the flame. It is 
unattacked by acids. It melts at about 1325 . Its powder reacts 
alkaline. 

It alters readily to albite, muscovite, eucryptite (LiAlSiCU), or mix- 
tures of these. One of the commonest mixtures is known as cymatclUe 
or cumaiolite. The mixture of albite and eucryptite has been called 
spodumene. 

Spodumene crystals have not been made artificially. 

Occurrence and Origin. — The mineral occurs in granites, pegmatites 
and crystalline schists, where it was formed by pneumatolytic processes. 
It is often associated with cassiterite. 

Localities. — Spodumene crystals occur at Huntington, Mass., in a 
quartz vein in mica schist; at Branch ville, Conn., in pegmatite; at 
Stony Point in Alexander Co., N. C, in cavities in a gneiss; at the Etta 
Mine and at other places in the Black Hills, N. D., in a pegmatite; at 
the lepidolite localities in California and in Minas Geraes, in Brazil. 

Uses. — The ordinary varieties of the mineral are used as a source of 
lithium in the manufacture of lithium salts, and the transparent varieties 



380 



DESCRIPTIVE MINERALOGY 



as gems. The total production of kunzite in this country during 191 2 
was valued at $18,000, all from California. One specimen found in this 
year weighed 47^ oz. Another was a crystal measuring 9X5X1 inches. 
The other forms of the mineral were not mined. In recent years a few 
tons have been furnished by the mines in the Black Hills. 

TRICL1NIC PYROXENES 

The triclinic pyroxenes include the four minerals rhodonite, bustamite y 
fowlerite and babingtonite. They are completely isomorphous. The 
first is the manganese metasilicate, MnSi03, and the others are iso- 
morphous mixtures of this molecule with the corresponding silicate of 
calcium (bustamite), or of these two with the corresponding iron (babing- 
tonite), or with the iron and zinc compounds (fowlerite). 

Rhodonite— Fowlerite (R"MnSi0 3 . R = Ca h Fe,Zn) 

Rhodonite is the pure manganese silicate with the percentage com- 
position shown in I. In II is the result of an analysis of crystals from 
Pajsberg, Sweden. An analysis of bustamite from Campiglia, Italy, is 
quoted in III and one of fowlerite from Franklin Furnace, N. J., in IV. 

Si0 2 AI2O3 MnO FeO ZnO MgO CaO H 2 Total 

.... 1 .65 

.... I . ol 

7-33 J -30 

All are triclinic (pinacoidal class), with the axial constants" of 

1.0729 : 1 : .6213, a=io3° 

i9',/3=io8 44',7 = 8i°39' 
for rhodonite, and 1.0807 : 1 

: .6237 and a=i02° 27', 
/3=io8°34 , ,7=82° 53' for 
babingtonite. Their crys- 
tals possess many habits, o£ 
which the cubical, tabular, 
and columnar are the most 
Fig. 208.— Rhodonite Crystals with 00 'P, 1T0 common. They are usually 
(Af); 00 p', no (w); oP, 001 (c); 00 p£, rough with rounded edges. 
100 (a); 00P 56, 010(6) ; 2 ,P, 221 (*) and 2P,, The most frequently oc- 

221 (n ^' curring forms are oP(ooi), 

00 P06 (100), 00 P6b (010), ooP'(no), oo'P(no), P,(nT) and 
2/P(22i) (Fig. 208). The angle iooAooi = 72° 37'. Their cleavage 



1. 45.85 


• • • 


54.15 


• • • • 


II. 45.86 


• • ■ 


4592 


36 


III. 49 23 


•37 


26.99 


1 .72 


IV. 46.06 


• ■ « 


3428 


3 *>3 





• • • * 


100.00 


6.40 


• • ■ • 


100.09 


18.72 


1-54 


100.38 


7.04 


.... 


99.64 





ANHYDROUS METASILICATES 381 

is perfect parallel to ooP'(no) and oo 'P(iTo). Although crystals are 
fairly common in some places, the minerals are more usually in dense, 
structureless or finely granular masses. 

All the triclinic pyroxenes have a glassy luster which is somewhat 
pearly on cleavage surfaces. They are transparent or translucent and 
all except babingtonite have a rose-red color when pure. When mixed 
with other substances their color may be yellowish, greenish, brownish 
or black. They are pleochroic in rose and yellowish tints. Their streak 
is always reddish white. Babingtonite is greenish black and is pleo- 
chroic in green and brown tints. All have an uneven fracture. Dense 
varieties are tough and their crystals are brittle. Their hardness 
= 5-6, and density 3.4-3.7. The intermediate refractive index of rhodo- 
nite is 1.73 for yellow light. 

Before the blowpipe all become black, swell and fuse to a brown 
glass. The fusing temperature of rhodonite is about 1200 and of 
bustamite about 1300 . They are attacked by acids with loss of color. 

When exposed to the weather the members of the group containing 
manganese alter to a mixture of which the principal constituents are a 
manganese oxide, Mn203, silica and water, or to mixtures of carbon- 
ates of manganese, or a mixture of the carbonates of manganese, iron 
and calcium. 

Syntheses. — Crystals of rhodonite have been prepared by fusing a 
mixture of SiC>2 and Mn02 and by passing a current of steam and CO2 
over a red-hot mixture of MnCfe and SiCfe. Rhodonite and babington- 
ite crystals are also formed in the slags of manganese iron furnaces, and 
the latter has been found in cavities in roasted iron ores. 

Occurrence. — The members of the group containing manganese occur 
in veins of magnetite, copper and other metals, and in contact zones 
between limestones, shales and igneous rocks, associated with other 
manganese minerals. Under these conditions they may have been pro- 
duced by the help of magmatic emanations. Rhodonite occurs also with 
rhodochrosite in deposits of manganese ores and in other associations, 
where it may be of secondary origin. Babingtonite occurs principally 
as a rare component of siliceous rocks. 

Localities. — Crystals of rhodonite and bustamite occur in iron ore 
deposits in the gneiss of Langban, Sweden. Fine crystals of rhodonite 
are found in the iron ore at Pajsberg, Sweden, and crystals of fowlerite 
in metamorphosed limestone associated with the zinc ores at Stirling 
Hill and Franklin Furnace, N. J. Massive rhodonite is abundant at 
Jekaterinburg, Ural, Russia; at Kapnik, Hungary; at Blue Hill Bay, 
Maine; and in Jackson Co., N. C, associated with wad. Massive bus- 



382 



DESCRIPTIVE MINERALOGY 



tamite occurs at Rfebanya, Hungary, in veins in limestone, and at Mts. 
Civillina and Campiglia, Italy, in fibrous masses. Babingtonite occurs 
in a mica schist at Athol, Mass., and in druses in granite at Baveno, 
Italy, and in the ore veins at Arendal, Norway. 

The principal occurrences of gem rhodonite in this country are in 
Siskiyou Co., Cal., and near Butte, Mont. In the former locality the 
mineral occurs nine miles north of Happy Camp in a fine-grained 
quartz schist. It consists of a mixture of quartz grains cemented by 
rhodonite and traversed by veins of pyrolusite. The Montana material 
is in radiating groups with quartz, pyrite and brown manganese oxide. 
At the Alice Mine it is associated with rhodochrosite. 

Uses and Production. — Transparent rhodonite is used as a gem-stone 
to a slight extent. The total yield of the material in the United States 
during 191 2 was valued at $550. 



THE AMPHIBOLES 

(R"SiO,, R'Al(Si0 3 ), and R // (R / "0) a Si0 4 ) 

The amphiboles are common alteration products of pyroxenes and 
some other silicates. They are also abundant as components of certain 
schistose rocks, as for instance, the hornblende schists, and they occur 
also as original constituents of igneous rocks. The crystals of all the 
amphiboles are similar in habit to those of the pyroxenes (Fig. 209), 
but since the ratio between the a and b axes is about .5 to 1, the angles 
between their cleavage planes, which, like those of the pyroxenes, are 
parallel to 00 P(no), are from 54 to 56 and 124 to 126 (see Fig. 
198B). The plane of symmetry bisects the obtuse angle. 

The members of the group are about as numerous as those of the 
pyroxenes, but the common types are much fewer. Moreover, there is 
no subgroup corresponding to the wollastonite subgroup of the pyrox- 
enes. The best known members of the series, with their axial ratios are: 



Anthophyllile 
Gedrite 



Orthorhombic (possibly twinned monoclinic). 

(Mg-Fe)SiOs ) a : b : c = .$2i : 1 : .2=4= 

(Mg.Fe)(A10) 2 Si04 j =.523 : 1 : .217 



Monoclinic (monoclinic prismatic class). 

Mg,Ca(SiO s )4 a : 6 : £ = .5415 : 1 ' .2886 

(Mg-Fe) 8 Ca(SiO,)4 
CummingtonUe (Fe • Mg) Si0 8 

FeSi0 3 

(Mg-Fe) 5 Ca(Si0 3 )« 

(Mg.Fc)((Al-Fe)0) s SiO« 

NaAl(SiO,) a 



Tremolite 
Actinoliie 



Griinerite 
Hornblende 



= 74° 49' 



.5318: 1 1.2937 = 75° 02' 



ANHYDROUS METASILICATES 



383 



Glaucophane 

Arfvedsonite 
RiebeckUe 

CrocidoliU 
AemgmaHte 



NaAl(SiO,)i 
(Fe.Mg)Si0 3 
(Na«.CaFe)SiO, 
(CaMg)((Al.Fe)0),SiO 
NaFe(SiO,) 2 
f NaFe(SiO,) t 
FeSiO, 



») 



*».53 : i : .29 

= .5496 : 1 : .2975 
- -5475 : 1 : .2925 



0~77 P 

= 75° 45' 
0» 76*10' 



Triclinic (triclinic pinacoidal class). 
Na4Fe 9 (AlFe) 2 (SiTi)uO» =.6778 : 1 : .3506 

ORTHORHOMBIC AM PHI BOLES 



= 7*°49' 



Anthophyllite — Gedrite 

The orthorhombic amphiboles are comparatively rare. They are 
isomorphous mixtures of MgSiC>3, FeSiOa and the alumino-orthosilicates 
(Mg» Fe)(A10)2Si04. The pure MgSiCta has not been found in nature, 
but it has been produced in the laboratory. The mixture of the mag- 
nesium and iron silicates (Mg* Fe)SiOs, is known as anthophyllite. In 
nature it always contains a little of the molecule (Mg*Fe)(A10)2Si04. 
Gedrite, which is much less common than anthophyllite, contains more 
AI2O3 than does this mineral, which may be regarded as due to a larger 
admixture of the molecule (Mg*Fe)(A10)2Si04. The name is thus 
applied to aluminous anthophyllites. 

The difference in composition of the two minerals is shown by the 
following analyses of (I) anthophyllite and (II) gedrite. 

Si0 2 Fe 2 3 AI2O3 MnO FeO MgO CaO Na 2 H2O Total 

I.57.98 ... .63 .31 10.39 28.69 .20 1.79 99.99 

II. 46.18 .44 21.78 ... 2.77 25.05 ... 



2 -3° I -37 99-89 



I. Brown crystals; Franklin, Macon Co., N. C. 
II. Colorless prisms; Fiskernas, Greenland. 



The orthorhombic amphiboles usually occur in platy or fibrous 
aggregates that rarely show traces of end faces, and, consequently the 
ratio between c and b is not accurately known. The planes in the pris- 
matic zone are, however, 1 sometimes so well developed that they 
can be recognized as ooP«(ioo), 00 P 06 (010), and ooP(no). 
Cleavage is perfect parallel to 00 P(no) and distinct parallel to 00 P 06 
(010). The cleavages intersect at angles 54 2o'-55° 18'. 

The minerals have a glassy luster which is slightly pearly on cleavage 
surfaces. They are green or brown in color and have a colorless, yellow 
white or gray streak and are translucent and pleochroic in colorless, 



384 



DESCRIPTIVE MINERALOGY 



greenish and brownish tints. Their fracture is somewhat conchoidal. 
Hardness is 5.5. and density 3.2. The refractive indices for yellow light 
in anthophyllite are: 0=1.633, s * 1.642, 7=1.657; and in gedrite, 
1.623, 1.636, and 1.644. 

Synthesis. — Pure magnesium metasilicate has been made in ortho- 
rhombic crystals mixed with monoclinic crystals, by rapid cooling of a 
magma made by heating Mg salts and silica with water at 375°-475° 

Occurrence. — The minerals are found in crystalline schists — more 
particularly in hornblende gneisses and hornblende schists, where they 
are distinctly metamorphic minerals, having been derived in some cases, 
at least, by the alteration of the orthorhombic pyroxenes. They alter 
to talc. 

Localities. — Anthophyllite occurs in dark brown platy aggregates at 
Kongsberg and Modum in Norway, associated with hornblende in mica 
schists; on the Shetland Islands, Scotland, associated with serpentine; 
and at the Jenks Corundum Mine in Macon Co., N. C. 

Gedrite occurs in yellowish gray fibrous aggregates at Bamle, Norway, 
in dark brown aggregates associated with magnetite and brown mica, at 
Gedres, Hautes-Pyr£n6es, France; and in a mica schist at Fiskernas, 
Greenland, associated with a large number of metamorphic minerals. 

MONOCLINIC AM PHI BOLES 

The monoclinic amphiboles, like the corresponding pyroxenes, com- 
prise isomorphous mixtures of the metasilicates of Na, Mg, Ca and Fe 



^> 



tn 



m 




e 



m 



m 





Fig. 209. — Ampibole Crystals with ooP, no (m); oopSb, 010 (b); 00 PJ, 130 («); 

P ob , on (r) and — P » , 101 (/). 



and the basic orthosilicates of Al and Fe. Recent work seems to indi- 
cate that in tremolite there is present also a little H2O. In the amphi- 
boles the alumino-silicate is more common than in the pyroxenes and 
consequently aluminous amphiboles are more common than aluminous 
pyroxenes. 



ANHYDROUS METASILICATES 385 

All the monoclinic amphiboles crystallize with the same habit in 
crystals that are columnar like those of the corresponding pyroxenes, 
but on which the terminations are different (Fig. 209). Moreover, all 
have a distinct cleavage parallel to ooP(no) with cleavage angles of 
about 56°-i24°. 

The amphiboles are distinguished from other minerals by their 
crystallization and their cleavage. 

For convenience, the monoclinic amphiboles may be subdivided into 
(1) the magnesium-calcium-iron amphiboles including tremolite, actino- 
lite, cummingUmite* grunerite and hornblende, and (2) the alkali amphi- 
boles, including arfvedsonite, glaucophane and riebeckite. 

Before the blowpipe all the members of the group fuse to a glass which 
is colorless, green or black, according to the quantity of iron present. 
The varieties rich in iron are attacked by acids. 

Magnesium-Calcium-Iron Amphiboles 

Tremolite-Hornblende 

This group includes the monoclinic amphiboles that are mainly meta- 
silicates of magnesium and iron and the mineral hornblende, which is a mix- 
ture of these molecules and the orthosilicate (Mg«Fe)((Al*Fe)0)2Si04. 
The calcium metasilicate is present in some members as an isomorphous 
mixture, but it does not occur alone as an independent member corre- 
sponding to wollastonite among the pyroxenes. Hornblende is the only 
member of the series that is essentially aluminous. 

The crystals of the monoclinic amphiboles are short columnar or 
long and acicular. Their axial ratios are nearly alike and their cleavage 
angles differ only by a few minutes. The simpler crystals are bounded 
by 00 Poo (100), 00 P co (010), ooP(no), oP(ooi), 3P 00(031), 
+P6o(Toi), — P 00(101), 2P2(T2i), 2P2(2ii) and Poo (on) (Fig. 
209). Contact twins are common, with 00 P 60 (100) the twinning 
plane as in the pyroxenes. Polysynthetic twins are rare. 

All the amphiboles of this group have a glassy luster and are trans- 
parent or translucent. All the members but hornblende are white or 
some shade of green, though colorless and brown varieties are not un- 
common and yellow and red varieties are known. Hornblende is fre- 
quently so dark as to be almost black. Their streak is light, hardness is 
5-6 and density 2.9-3.4, depending upon composition. 

The cleavage is perfect in all the amphiboles and there is present 
often also a parting parallel to 00 P do (100) and P 00 (Toi), the latter due 
to gliding. Pleochroism is strong in all the colored varieties in green 



386 



DESCRIPTIVE MINERALOGY 



and yellowish green tones in the green varieties, and brown and yellow- 
ish brown tints in the brown varieties. 

Tremolite is the calcium magnesium silicate. When there is mixed 
with this the corresponding iron molecule the mixture is known as 
actinolite if the proportion of the iron molecules present is not great. 
The theoretical compositions of the two molecules Mg3Ca(Si03)4 and 
Fe3Ca(Si03)4 are given in lines I and II, and analyses of several trem- 
olites and actinolites in lines III, IV, V and VI. The almost universal 
presence of small quantities of water in tremolite, and the lack of 
enough Mg, Ca, Fe and other metallic bases to satisfy all the SiCfe re- 
vealed by the analyses has suggested to some mineralogists that the 
water is an essential part of the compound, and that its composition is 
best represented by MgsCa2H2(Si03)8. 



Si0 2 AI2O3 Fe 2 3 FeO MgO CaO Na 2 H 2 



I- 57 -72 


• • • 


II. 46.90 


■ • * 


III. 58.27 


•33 


IV. 57 40 


•38 


V. 58.80 


• • • 


VI. 55 50 


■ ■ ■ 



tr. 



28.83 13 

42.17 10 

25.93 11 

25.69 

22.23 

22.56 



1.36 

3 05 
6.25 



13 
16 



13 



45 

93 
90 

89 

47 
46 



25 



1.22 
.40 

• « • • 

1.29 



Total 
100.00 
100.00 
99.40* 
99.12 

i°°-55 
99.06 



I. Theoretical for MgjCa (SiO?)4. 
II. Theoretical for Fe s Ca (SiOa)^ 

III. Tremolite, East on, Pa. 

IV. Tremolite, Gouverneur, N. Y. 
V. Asbestus, Bolton, Mass. 

VI. Actinolite, Greiner, Zillerthal, Tyrol. 

* Also .08 MnO and .42 K 2 0. 

Tremolite is white or nearly white, and actinolite is green. The 
former occurs in columnar crystals, in plates and occasionally in fibers, 
while actinolite is nearly always in long, slender acicular crystals without 
terminations. The refractive indices for yellow light in tremolite are: 
a= 1.6065, £=1.6233, 7=1.6340. In actinolite, a =1.61 1 6, = 1.6270, 
7=1.6387. 

Both minerals melt in the blowpipe flame, the fusing temperature 
for tremolite being about 1290 and for actinolite about 1150 . 

Asbestus is a fibrous variety of tremolite, actinolite or anthophyllite. 
It occurs principally in rocks that have been crushed and sheared under 
great pressure. The actinolite asbestus is used for the same purpose as 
the chrysotile variety (see p. 398), but it is regarded as less valuable. 



ANHYDROUS METASILICATES 387 

Its principal source in this country is Sails Mountain, Georgia, but prom- 
ising deposits have recently been reported near Kamiah, Idaho. At the 
Georgian locality the asbestus forms distinct lenses in gneiss. It is 
possibly an altered basic intrusive rock. 

Smaragdite is a grass-green actinolite, which is often an alteration 
product of pyroxenes and olivine. The name is also applied to a bright 
green hornblende containing a little chromium. 

Nephrite is a finely fibrous actinolite or tremolite and usually some 
chlorite, forming dense rock masses that are white or of a light green 
color. It was formerly much used, like jade, in the manufacture of 
images, charms and implements. 

Cummingtonite and griinerite are amphiboles containing notable 
quantities of the molecule FeSiQj. In griinerite, the quantity of Mg 
present is very small but in cummingtonite it is fairly large. Because 
of its similarity to anthophyllite, this mineral is frequently referred 
to as amphibole-anthophyllite. It is intermediate in composition 
between griinerite and actinolite. Analyses of specimens from several 
well known localities are quoted below. 

FeO MgO CaO Na 2 H 2 Total 

15.64 21.70 tr. 2.80 99.88 

43.40 2.61 1.90 .47 2.22 100.08 

I. Cummingtonite, near Baltimore, Md. 

II. Griinerite, Collobrieres, France. Contains also, F=.o7, KfO=*.07 and 
MnO = .o8. 

These two minerals are comparatively rare and have not always 
been recognized as worthy of different names. In general appearance 
they are much like actinolite, though perhaps more brown or gray in 
color, and they occur in nearly the same association. The specific grav- 
ity of cummingtonite varies between 3.1 and 3.3 and that of griinerite is 
about 3.52. The intermediate refractive index for yellow light is 1.62- 
1.65 in cummingtonite and 1.697 m griinerite. 

Hornblende is the name given to the monoclinic aluminous amphi- 
boles that contain only a small quantity of alkalies. In other words, 
most of the hornblendes are isomorphous mixtures of the actinolite mole- 
cule and the molecules (Mg- Fe)((Al- Fe)0) 2 Si0 4 and (Na- K)Al(Si03) 2 . 
The varieties containing Na 2 (known as katofarite) correspond to 
aegirine among the pyroxenes. 



Si0 2 


A1 2 3 


Fe 2 C>3 


I. 57.26 


•75 


i-73 


II. 47.17 


1. 00 


1. 12 



fi 


y 


1 .620 


1.632 


1 .642 


1 653 


1-725 


1-752 



388 DESCRIPTIVE MINERALOGY 

The varieties of hornblende that are distinguished by distinctive 
names are: 

Pargasite, the green, bluish green or greenish black variety, and 

Edenite, the white, gray or light green variety, both of which con- 
tain very little iron in either the ferrous or ferric condition, 

Smaragdite, a bright green chromiferous variety of pargasite, 

Common Iwrnblende, the greenish black variety, 

Basaltic hornblende, which contains a large proportion of ferric iron 
and is black in color. 

Their refractive indices for yellow light are as follows: 

a 

Pargasite, Pargas, Finland 1 . 613 

Common Hornblende, Kragero, Norway 1 . 629 
Basaltic hornblende, Bohemia 1 . 680 

The fusing temperature of pargasite is about 1150 and of horn- 
blende about 1200 . 

Analyses of typical specimens of these varieties follow: 

Si0 2 AI2O3 Fe 2 3 FeO MgO CaO Na 2 K 2 Ign Total 

I. 5169 4.17 2.34 9.83 17.17 12.17 .82 .79 1. 13 100.25 

II.42.97 16.42 1.32 20.14 x 4-99 i-53 2-85 .87 102.75 

III. 49.33 I2 -72 172 463 17.44 9.91 2.25 .63 .29 99.13 

rv. 39.17 14 .37 12.42 5.86 10.52 11. 18 2.48 2.01 .39 99.91 

I. Common Hornblende, Vosges. Also .14 per cent TiCfe. 
II. Pargasite, Pargas, Finland. Also 1.66 per cent F. 

III. Edenite, Saualpen, Carinthia. Also 1.21 per cent F. 

IV. Basaltic, Jan Mayen, Greenland. Also 1.51 per cent MnO. 

Among the commonest forms of alteration in the amphiboles 
are the following: Tremolite into talc (p. 401) and serpentine, and 
hornblende into serpentine, chlorite (p. 428), epidote and biotite, often 
with the addition of magnetite and other iron compounds in cases where 
iron was present in the original mineral. Most of these changes are 
brought about by regional metamorphism. The production of biotite is 
also brought about by the action of magmas. The common weathering 
products of hornblende are chlorite, epidote, calcite, quartz, magnetite 
and siderite. Under the conditions of high temperature and high pres- 
sure, hornblende sometimes passes over into augite and magnetite. 

Syntheses, — Amphibole crystals have not been found in slags nor 
have they been made by dry fusion. Crystals of hornblende, however, 



ANHYDROUS METASILICATES 389 

have been obtained by heating to 555 ° for three months, a mixture of 
its components in a glass tube with water. 

Occurrence. — Tremolite occurs in crystalline limestones and dolo- 
mites that have been subjected to regional metamorphism and in crys- 
talline schists. Actinolite, cummingtonite and griinerite are found in 
crystalline schists, in some cases in such large quantity as to constitute 
essential parts of the rocks. Actinolite schists are such rocks containing 
in addition to the actinolite some quartz, epidote and chlorite. Grii- 
nerite schists consist essentially of griinerite, actinolite, magnetite and 
quartz. 

Common hornblende occurs in igneous and metamorphic rocks, 
such as gneisses and schists. In some schists, as the amphibolites, 
it is the principal constituent and in others, the hornblende schists, 
it is the principal component other than quartz. The mineral is 
also a common metamorphic alteration product of pyroxenes which 
it frequently pseudomorphs. When the pseudomorphing hornblende 
is blue-green and fibrous it is known as uralite. The chemical 
changes attending this alteration are illustrated by the analyses of a 
pyroxene (I) from the Grua Tunnel in Norway and of the uralite (II) 
produced from it. 

Si0 2 Fe 2 3 AI2O3 FeO MnO CaO MgO Na 2 Loss Total 
I. 50.53 1. 91 .27 7.81 1.99 24.51 10.92 .48 .26 100.37* 
II. 42.02 2.30 3.25 9.30 .94 20.90 9.63 .45 1.07 100.04* 

* Also .19 per cent K 2 in I and .26 per cent in II. 

Basaltic hornblende is found only in igneous rocks, and especially 
those rich in iron. 

Edenite occurs in crystalline limestones that have been metamor- 
phosed by contact action. 

Pargasite is in gneisses and crystalline limestones. 

Localities. — Tremolite crystals occur at Campolonga, Switzerland; 
at Rezb&nya, Hungary; at New Canaan, Conn.; and at Diana, Lewis 
Co., N. Y. It occurs also in flat plates at Lee, Mass.; near Byram, 
N. J.; at Easton, Penn. ; at Edenville, N. Y.; and at Litchfield, Me., 
and other places in the limestones in Quebec, Canada. 

Actinolite occurs with chlorite at the Zillerthal, Tyrol; in talc and 
chlorite schists near Jekaterinburg, Ural, Russia; at Arendal, Norway; 
at Willis Mt., Buckingham Co., Va.; at the Bare Hills, Md.; at Mineral 
Hill, in Delaware Co., and at Unionville, Penn., in the soapstone 
quarries at Windham and New Fane, Vt.; at Bolton, Brome Co., 
Quebec, and at many other points. 



390 DESCRIPTIVE MINERALOGY 

Asbestus is abundant at Sterzig, in Tyrol; on the Island of Corsica; 
near Greenwood Furnace, N. Y.; in the Bare Hills, near Baltimore, Md. ; 
at Pylesville, Harford Co., in the same State; at Barnet's Mills, Fau- 
quier Co., Va., and at the localities at which it has been mentioned as 
being mined. 

The principal occurrences of cummingtonite are Kongsberg, Norway, 
Cummington, Mass.; and a layer in gneisses and schists at Mt. 
Washington, Md. 

Griinerite occurs in a rock composed of this mineral, garnet and hem- 
atite near Collobrifcres, Var., France. It has also been described as the 
principal constituent of certain schists in the Lake Superior iron region, 
but since the amphibole in these rocks contains a notable quantity 
of MgO it should better be classed with cummingtonite. 

The localities at which crystals of the hornblendes have been 
found are very numerous. Excellent crystals occur in the volcanic 
bombs in the Lake Laach district, Prussia; in cavities in inclusions 
within the lavas of Aranyer Mt., Siebenburgen, Hungary; in the dikes 
of porphyry, near Roda, Tyrol; on the walls of cavities in inclusions in 
the lavas at Vesuvius, Italy; and at various points in Sweden, etc. . In 
North America fine crystals are found at Thomaston, Me.; at Russell 
and Pierrepont, N. Y.; at Franconia, N. H.; and in the glacial debris 
at Jan Mayen, Greenland. Pargasite occurs at Pargas, Finland, and 
Phippsburg, Me. 

Alkali Amphiboles 

The alkaline amphiboles include riebeckilc, crocidolite, glaucophane 
and arfvedsonile. The first two are nonaluminous iron-soda amphiboles 
and the last two are aluminous compounds. Glaucophane contains the 
molecule NaAl(Si03)2 which is found also in hornblende, and, therefore, 
it may be regarded as a connecting link between the common and the 
alkaline amphiboles. Glaucophane differs from hornblende, however, 
in containing very little CaO. The intermediate link katoforite bridges 
the gap between the two. 



Glaucophane 



NaAl(Si0 3 ) 2 
(Fe-Mg)SiOa t 



Glaucophane is, theoretically, a mixture of the two molecules 
NaAl(Si03)2 and (Fe* Mg)Si03. It is essentially a mixture of the cum- 
mingtonite molecule with one corresponding to the jadeite molecule 



ANHYDROUS METASILICATES 391 

among the pyroxenes. An analysis of a specimen of katoforite (com- 
pare p. 387) from the sanidinite bombs in the lava at Sao Miguel, Azores, 
is quoted in line I for comparison with the two glaucophane analyses in 
lines II and III. 



Si0 2 


AI2O3 Fe 2 03 


FeO 


MgO 


CaO 


Na 2 


K 2 


Total 


I- 45 • 53 


410 9-35 


23.72 


2.46 


4.89 


6.07 


.88 


99.96 


n. 56.65 


12.31 3.01 


458 


12.29 


2. 20 


7-93 


IO S 


100.02 


III. 56.71 


15.14 9.78 


4-3 1 


4-33 


4.80 


4.83 


■25 


100.15 



I. Katoforite, Sao Miguel, Azores. Also 2.96 per rent TiO*. 
II. Glaucophane, He de Groix. 
III. Glaucophane, Shikoku, Japan. 

Glaucophane is rarely found in crystals with end faces. Even when 
these exist they are rough and yield poor measurements. 

The mineral occurs in columnar crystals, in needles and in foliated 
or granular aggregates in rocks. Their prismatic planes are 00 P 60 (100), 
00 P ob (010) and 00 P(no). P(Tn) and oP(ooi) are the only termina- 
tions that have been identified. The cleavage angle is about 55 ° 20'. 

Glaucophane is blue or bluish black, translucent and strongly pleo- 
chroic in yellowish, violet and blue tints. Its streak is grayish blue, 
its fracture uneven, its hardness about 6 and its density 3. Its refractive 
indices for yellow light are: a= 1.62 12, 0= 1.6381, 7= 1.6300. 

Before the blowpipe the mineral turns brown and then melts to an 
olive-green glass. It is difficultly attacked by acids. 

Glaucophane is distinguished from the other amphiboloids by its 
color, and from other blue silicates by its crystallization, hardness and 
manner of occurrence. 

It is usually unaltered but it has been described in one instance as 
being partially changed to chlorite. 

Synthesis. — It has not been produced artificially. 

Occurrence. — The mineral is found only in metamorphosed limestones, 
in mica schists and in the garnet rock known as eclogite. It is charac- 
teristically a metamorphic mineral. 

Localities. — Glaucophane occurs in long crystals in various schists in 
Syra, Cyclades, Greece; in hornblende schists in the He de Groix, Brit- 
tany, France; in a glaucophane schist on the Island of Shikoku, Japan; 
and abundantly in various SJ Aists in the Coast Ranges of California. 



\ 



392 DESCRIPTIVE MINERALOGY 



Arfvedsonite, Riebeckite and Crocidolite 

These amphiboles are comparatively rare. They occur principally 
in coarse-grained alkaline igneous rocks, usually as prismatic grains 
without terminations, embedded in the rock mass. Arfvedsonite, how- 
ever, in some cases, occurs in groups of crystals on some of which ter- 
minations can be identified. 

Riebeckite, NaFe(S • 03)2, has a composition very near that of acmite, 
and crocidolite contains, in addition, the molecule FeSi03. Arfvedsonite 
is much more complex than either of these and has no equivalent among 
the pyroxenes. Analyses of typical specimens of the two minerals are 
quoted below. In line IV is an analysis of crocidolite. 

Si0 2 AI2O3 Fe 2 3 FeO MgO CaO K 2 Na 2 H 2 Total 



I. 47.08 1.44 1.70 


35-65 


... 2.32 


2.88 7.14 


2.08 


100.29 


n. 49.65 1.34 17.66 


19-55 


... 3. 16 


.... 7.61 


1.67 


100.64 


in. 50 . 01 .... 28 . 30 


9.87 


•34 i-3 2 


.72 8.79 


• • * • 


99.98 


rv. 5103 17.88 


21 . 19 


.09 .... 


.... 6.41 


364 


100.24 



I. Black arfvedsonite, Kangerdluarsuk, Greenland. 

II. Riebeckite from granite, Quincy, Mass. 

III. Riebeckite from Socotra, Indian Ocean. 

IV. Dark blue radial aggregates of crocidolite, Cumberland, R. I. 

Arfvedsonite is usually in long prisms flattened parallel to 
00 P 00 (010), but otherwise very much like hornblende. It is black or 
dark green and translucent, and has a dark bluish gray streak. Its hard- 
ness is 6 and density 3.4-3.5. It is strongly pleochroic. Thin splinters 
parallel to 00 Poo (010), are olive green and those parallel to 00 P 6b 
are deep greenish blue. Its refractive indices for yellow light are: 
a= 1.687, 18=1.707, 7=1.708. 

Before the blowpipe the mineral fuses easily to a black magnetic 
globule and colors the flame yellow. It is not acted upon by acids. . 

Riebeckite is found only in embedded prisms, showing no termina- 
tions. It is black, vitreous and very pleochroic in green and dark blue 
tints. Its density is about 3.3, and its hardness 5.5-6. Its refractive 
index for yellow light is about 1.687. Before the blowpipe it fuses easily, 
imparting an intense yellow color to the flame. 

Crocidolite is an asbestus-like, lavender-blue or dark green riebeckite, 
that contains a larger amount of iron, due to the presence of the mole- 
cule FeSiC>3. It occurs also in earthy masses. Its streak is lavender-blue 
or leek-green and its hardness is 4. In all cases it appears to be a 
secondary mineral. The green fibrous variety is known as " cat's-eye." 



ANHYDROUS METASILICATES 393 

Both riebeckite ancU-rfvedsonite weather to aggregates of iron oxides, 
quartz and carbonates. The decomposed, brown crocidolite is the well- 
known ornamental stone " tiger's-eye." 

Occurrence and Localities. — Arfvedsonite is found principally in 
igneous rocks rich in soda, especially the coarse, nepheline syenites of 
Greenland; Kola, Russia; and in the augite syenites of Norway. It 
occurs also in the nepheline syenites of Dungannon township, Ontario, 
and of the Trans-Pecos district, Texas. 

Riebeckite is also formed in acid rocks rich in soda, such as certain 
granites, syenites, etc. It is found on the Island of Socotra in the Indian 
Ocean; in fine-grained granitic rocks at Ailsa Crag, Scotland; in Corsica 
and a few other places. The crocidolite variety occurs in a clay slate 
on the banks of the Orange River in South Africa; at various points in 
the Vosges, Salzburg, Tyrol and Andalusia, in Europe; in Templeton, 
Ontario; in veins at Beacon Pole Hill, near Cumberland, R. I., in gran- 
ites at Quincy and Cape Anne, Mass., near St. Peter's Dome, El Paso 
Co., Colorado, and as fibers in rocks at various other points in the United 
States. 

TRICLINIC AM PHI BOLE 

The only known triclinic amphibole is the comparatively rare aenig- 
matite, an alkali amphibole with a complicated composition that may 
be represented by the formula Na4FeQ(Al*Fe)2(Si*Ti)i2038. The 
mineral occurs in very complex crystals, with iioAiTo=66°, in 
alkaline rocks at Naujakasik, Greenland; in the Fourch Mts., Ark.; 
and at several other places. 

It is black, or brownish black, and translucent or transparent and 
has a reddish brown streak. It is, moreover, strongly pleochroic in 
brownish black and reddish brown tints. It is brittle, has a hardness 
of a little more than 5 and a density of 3.7-3.8. Before the blowpipe it 
fuses to a brownish black glass. It is partly decomposed by acids. It 
is distinguished from other dark hornblendes by the cleavage angle 
of 66°. 

BASIC METASILICATES 

Kyanite ((A10) 2 Si0 3 ) 

Kyanite, cyanite, or disthene, is a fairly common product of meta- 
morphism in certain schists. The name kyanite suggests the sky blue 
color noticed in many specimens. The name disthene refers to the 
great difference in hardness exhibited in different directions. 

The mineral is regarded as a basic metasilicate of the theoretical 



394 



DESCRIPTIVE MINERALOGY 



composition: Si02=37-02; Al203=r62.98 (compare pages 319, 320). 
Nearly all specimens contain a little Fe203 but otherwise they cor- 
respond very closely to the calculated composition indicated by the 
above formula. A light blue specimen from North Thompson River, 
B. C, upon analyses, gave: 



S1O2 
36.29 



AI2O3 

62.25 



Fe203 

♦55 



CaO 

1.06 



MgO 

•36 



Total 
100.51 




Fig. 210. — Kyanite Crys- 
tal with 00 P * , 100 (a); 

00 Poo, 010 (6); oP, 
001(c); 00 'P, 1 To (if); 

00 P', no (m) and 

00 P'2, 210 (/). 



Kyanite crystallizes in the triclinic system (triclinic pinacoidal 
class), with an axial ratio .8991 : 1 : .7090; a = 90° 55', 0=ioi° 2' and 
7=105° 445'. Very few crystals are well developed. Their habit is 

columnar or tabular with 00 P 60 (100) predomi- 
nating. More frequently the mineral occurs 
in long, flat, isolated blades, or in diverging 
flat plates (Fig. 210). Some crystals are 
very complex. Usually, however, only the 
forms ooP<»(ioo), 00 P 60 (010), ooP'(no), 
00 'P2~(2io), 00 , P(iTo) and oP(ooi) are pres- 
ent. Twinning is common according to several 
laws, most of which, however, yield twins in 
which the basal planes (oP) of the twinned in- 
dividuals are parallel. The most frequent 
twins have 00 P rc (100) as the twinning plane. 
Other twinning planes are perpendicular to the axis c, or to the 
axis b. The basal plane oP(ooi) also serves as the twinning plane 
in some cases. Twinning is often repeated, producing lamellae crossing 
columnar crystals approximately parallel to the basal plane, and giving 
rise to a definite parting in this direction. 

The cleavage of kyanite is very perfect parallel to 00 P 66 (100) 
and less perfect parallel to 00 P 06 (010). It frequently possesses also a 
parting parallel to oP(ooi), as already stated. The luster on cleavage 
faces is pearly. Otherwise it is glassy. The mineral is often light blue 
in color, less frequently it is colorless or white, yellow, green, brown or 
gray. It is translucent or transparent and the darker blue varieties are 
pleochroic in dark and light blue tints. Its hardness varies greatly 
on different faces and in different directions on the same face. On the 
macropinacoid a it is about 5 parallel to the vertical edges, and 7 in the 
direction at right angles to this. The specific gravity of the mineral is 
about 3.6, and its refractive indices for yellow light are: a^i.7171, 
0=1.7222,7=1.7290. 

Before the blowpipe kyanite whitens, but otherwise it reacts like 



ANHYDROUS METASILICATES 395 

sillimanite. It is insoluble in acids. It is distinguished from the few 
other minerals that it resembles by the great differences in hardness on 
its cleavage surfaces. At a high temperature (about 1350°) it appar- 
ently changes to sillimanite. 

Kyanite weathers to muscovite, talc (p. 401) and pyrophylUte 
(p. 406), and is itself an alteration product of andalusite and corundum. 

Syntkesis. — It is not known that the mineral has been produced in the 
laboratory. 

Occurrence and Origin. — Kyanite occurs as large plates and small 



Fir,, an.— Bladed Kyanite Crystals in a Miraienus Quartz Schist from Pisuso Fomo, 

Switzerland. (About natural size.) 

crystals in micaceous and other schists (Fig. an), and as an important 
constituent of some quartzites. At Horrsjoherg, in Wermland, Sweden, 
it forms a distinct layer of schist several meters thick. In a few places 
it is found in zones of contact metamorphism, but it is more frequently 
the result of dynamic metamorphism (cf. p. 26). 

Localities. — Crystals have been found at Greiner in the Tyrol; at 
Mte. Campione in Switzerland; and at Graves Mt. in Lincoln Co., Ga. 
The mineral also occurs in fine plates at Chesterfield, Mass.; at Litch- 
field, Conn.; at Bakersville, N. C; and on North Thompson River, 
B. C, Canada. 

Uses. — Transparent kyanite is sometimes used as a gem. 



396 



DESCRIPTIVE MINERALOGY 



ZnO 


H 2 


Total 


67.49 


7-5o 


100.00 


67.88 


813 


99.96 


65-°5 


7.89 


99 38 



Calamine ((ZnOH) 2 Si0 3 ) 

Calamine, or hemimorphite, is an important ore of zinc. It is one of 
the few silicates used as a source of metals. While theoretically a 
pure zinc compound it usually contains a little Fe2<33 and frequently 
small quantities of PbO. In some cases it contains also a little carbon- 
ate. A number of formulas have been suggested for it, of which the 
one given above is the simplest. According to several prominent miner- 
alogists, however, the formula Z^SiO* • H2O is preferable. 

Si0 2 Fe 2 03 

Theoretical 25 .01 

Wythe Co., Va 23 . 95 

Friedensville, Pa 24.32 2.12 

The mineral occurs in brilliant crystals that are orthorhombic and 
distinctly hemimorphic (rhombic pyramidal class), with an axial ratio 

of .7834 : 1 : .4778. The crystals 
are usually tabular parallel to 
00 P 06 (010). Many are highly 
modified but some are fairly sim- 
ple, with 00 P(i 10) , 00 P 00 (100) 
and 3P<x>(30i) in the pris- 
matic zone, 3P06 (031), P66 (101), 
P06 (on) and oP(ooi) at the ana- 
logue pole and 2P2(i27) at the 
Fig. 212.— Calamine Crystals with 00 P, antilogue pole (Fig. 212). The angle 
no (m); 00P 00 L 100 (a); 00 P 06 , IIO A lTo= ?6 ° Q / # Twins are fairly 
010(b): 2P2, 121 (t»); Poo,ioi (5); ... -p., N , . 

„ w ' „ _ , t [ n J common, with oP(ooi) the twinning 

Poo, on (e); 3 Pqo, 3 oi (/); 3P00, ' 4 v ; 6 

031 (1) and oP, 001 (c). P lane * # 0ften manv oystals are 

grouped in sheaf-like, fibrous or 

warty aggregates and in crusts. The mineral is also granular and 

compact. Its cleavage is perfect parallel to 00 P(no). 

Calamine is glassy, transparent or translucent, and when pure is 
colorless or white. Usually, however, it is gray, yellow, brown, greenish 
or bluish. Its streak is white, its hardness 4-4.5 an d its density 3.2-3.5. 
It is brittle. Its fracture is uneven. The mineral is strongly pyroelectric 
with the end of the crystals terminated by dome faces the analogue pole. 
In contact twins both ends are analogues. The mineral becomes phos- 
phorescent upon rubbing, and is fluorescent in ultra violet light. Its 
refractive indices for yellow light are: #=1.6136, 0=1.6170, 7=1.6360. 

Before the blowpipe calamine is almost infusible, but on charcoal 
it swells, colors the flame greenish and fuses with difficulty on the edges. 





ANHYDROUS METASILICATES 397 

With soda it gives the zinc sublimate. In the closed glass tube it de- 
crepitates and yields water and becomes cloudy. Its powder dissolves 
in even weak acids with the production of gelatinous silica. 

Calamine is distinguished from smithsonite by its reaction with acids 
and from other minerals by its crystallization and reaction for zinc. It 
alters to willemite, smithsonite and quartz. Calamine has not been 
produced artificially. 

Occurrence. — It occurs principally in the upper or oxidized zones of 
veins of zinc ore and in layers above the zone of permanent ground water 
in certain zinc and lead-bearing limestones. It is associated with lead 
ores and various zinc compounds, and it often pseudomorphs calcite, 
galena and pyromorphite. 

Localities. — Calamine occurs in nearly all places where zinc and 
lead ores are found. It is abundant at Altenberg near Aachen in Rhen- 
ish Prussia; at Wiesloch, in Baden; near Tamowitz, in Silesia; at 
Rezb&nya, Hungary; near Bleiberg, Carinthia; near Santander, Spain; 
in Cumberland, England; at Sterling Hill, N. J.; at Friedensville, 
near South Bethlehem, Penn.; at the Bertha Mine in Pulaski Co., and 
at the Austin Mine, in Wythe Co., Va.; and in the zinc-producing areas 
in the Mississippi Valley. 

Uses. — It is a common associate of other zinc ores and many lead 
ores and is mined with the former as a source of zinc. 

ACID METASILICATES 

SERPENTINE GROUP 

The serpentine group includes a large number of hydrous magnesium 
silicates that differ from one another mainly in the proportions of water 
present and in the ratio of silica to magnesia. None of them yields 
crystals, though their crystallization is thought to be monoclinic. All 
occur in dense fibrous or platy aggregates. The most prominent mem- 
bers of the group are: 

Serpentine H4Mg3Si2C>9, or SiC>2 MgO H2O 

H(MgOH) 3 (Si0 3 )2 =43 50 43-46 13. 04 

Meerschaum HiMg2Si30io, or 

H 3 Mg(MgOH)(Si03)3 =60.83 27.01 12.16 

Steatite H2Mg3(Si03)4 =63. 52 3172 4. 76 

All are soft and nearly infusible, and all are of considerable economic 
importance. 



398 DESCRIPTIVE MINERALOGY 

Serpentine (H4Mg 3 Si 2 Oo) 

The substance known as serpentine may be two different minerals, 
one orthorhombic and the other monoclinic. They, however, cannot 
be distinguished, except by microscopic study. Serpentine occurs in 
structureless, fibrous, foliated and schistose masses of a white, gray, 
brown or green color. It is translucent and has a dull, slightly glistening 
or fatty luster, and a white streak. The variety known as " noble ser- 
pentine " is nearly transparent and has a clear greenish or yellowish 
white, yellowish green, apple-green or dark green color. The mineral, 
when pure, has a hardness of 3, but it frequently seems harder because 
there are often mixed with it tiny remnants of the much harder minerals 
from which it was derived. The specific gravity of pure serpentine is 
2.5-2.6. Its refractive indices vary widely. /3=i. 502-1. 570. 

Serpentine fuses on thin edges when heated in the blowpipe flame. 
It yields water in the closed tube. When heated to about 1400 it crys- 
tallizes as olivine. It is decomposed by hydrochloric and sulphuric 
acids with the separation of gelatinous silica, which, in fibrous varieties, 
retains the shapes of the fibers. It is also soluble in dilute carbonic acid. 
Its powder reacts alkaline. 

Ckrysotile is a silky, nearly transparent fibrous variety occurring in 
veins. It is apparently orthorhombic. 

Antigorite is a form occurring in laminated masses or in microscopic 
scales, that are possibly monoclinic. 

Baltimorite and picrolite are coarse, green, fibrous varieties. 

Analyses of a pure green serpentine, and a typical chrysotile, both 
from Montville, N. J., are quoted below: 



Si0 2 


AI2O3 


Fe2C>3 


FeO 


MgO 


CaO 


H 2 


Total 


I. 42.05 


• • • 


•30 


.10 


42.57 


•05 


14.66 


99-73 


II. 42.42 


.63 


.62 


• • • 


41.01 


■ • • 


15-64 


100.55 



I. Green serpentine, Montville, N. J. 
II. Chrysotile, Montville, N. J. Also .23 NiO. 

Massive varieties are distinguished from talc by their solubility in 
acids and by differences in hardness, and chrysotile is distinguished from 
ampkibole asbestus by the presence in it of water. 

Synthesis, — Serpentine has been made by the action of a solution of 
Na2Si03 upon magnesite for 10 days at ioo°. 

Occurrence. — The mineral is a common decomposition product of 
several other magnesium silicates, more particularly olivine, pyroxene 



ANHYDROUS METAS1LICATES 399 

and chondrodite. Many igneous rocks rich in these minerals are com- 
pletely changed to serpentine, especially around their peripheries, and 
some metamorphosed limestones are also partially or completely ser- 
pentinized. It is probably a secondary mineral in all cases. 

Localities. — Serpentine occurs in large quantity at Webster, N. C; 
Montville, N. J.; Easton, Penn.; at the Tilly Foster Iron Mine, 
Brewster, N. Y., at Thetford and Black Lake in the Eastern Townships 
of Quebec, and at many other places in North America. It is also known 
from many places in Europe. 

Uses. — Serpentine when massive is used as a building stone. The 
finer varieties are sawed into thin slabs and used for ornamental purposes. 
Marble with streaks and spots of serpentine is known as ophicalcite. and 
under the name "verd-antique " is employed as an ornamental stone. 
Mixtures of serpentine with other soft minerals are ground for a paper 
pulp. The fibrous variety — chrysotile — is mined and sold under the 
name of asbestos, which, because of its fibrous structure, its flexibility, 
its incombustibility, and because it is a nonconductor of heat and 
electricity is becoming an exceedingly important economic product. It 
is woven into paper and boards that arc used to cover steam pipes, and 
to increase electric insulations, and is manufactured into shingles. It 
is used also in fireproofing, in the manufacture of automobile tires, 
in making paints, and as a substitute for rubber in packing steam 
pipes. 

Preparation. — The chrysotile mined in Vermont comes from a mass 
of serpentine that is cut by many small veins of chrysotile. The rock is 
crushed and the fiber is separated by washing, or by some other mechan- 
ical method. The pulp rock at Easton is a mass of serpentine, talc and 
a few other minerals. It is ground and sized for use in paper manu- 
facture. 

Production. — Chrysotile is mined in Vermont and Wyoming. The 
production Is rapidly increasing but the actual amount mined annually 
has not been disclosed. The total aggregate of chrysotile and amphibole 
asbestos (see p. 386), produced in the United States during 191 2 was 
4.403 tons, valued at $87,959. The imports of unmanufactured asbestos 
for the same year were valued at $1,456,012, of which $1,441,475 worth 
came from Canada. The total production of this country in the same 
year amounted to about $2,979,384, most of which came from the Thet- 
ford district in Quebec. This is about 80 per cent of the world's pro- 
duction. The value of the serpentine used as an ornamental and build- 
ing stone is not known. 



400 DESCRIPTIVE MINERALOGY 



Garnierite may be regarded as a serpentine or talc in which a portion 
of the magnesium has been replaced by nickel, or possibly as a mixture 
of a colloidal magnesium silicate and a nickel compound. Its impor- 
tance consists in the fact that it is the only commercial source of nickel 
aside from the pentlandite in the pyrrhotite of Sudbury, Canada. 
Three analyses of garnierite from New Caledonia follow: 



Si0 2 


NiO 


MgO 


A1 2 • Fe 2 03 


H 2 


Total 


35-45 


45.15 


2.47 


•50 


15-55 


99.12 


3778 


. 3391 


10.66 


i-57 


15-83 


9975 


42.61 


21.91 


18.27 


.89 


1540 


99.08 



These show that as MgO diminishes, NiO increases. 

Garnierite is a dark green to pale green substance with many of the 
physical properties of serpentine. Its luster is dull, or like that of var- 
nish. It has a greasy feel, a hardness of 2-3 and a density of 2.3-2.8. 
Its streak is light green to white. When touched to the tongue it ad- 
heres like clay. It is infusible when heated before the blowpipe, but 
decrepitates and becomes magnetic. It is partly soluble in HC1 and 
HNO3. 

It is readily distinguished from malachite and chrysocolla by its 
structure, its greasy feel and the absence of a good copper test. 

Occurrence and Localities. — The mineral occurs as earthy masses, as 
mamillary coatings and as impregnations and veins in serpentine. In 
all cases it appears to have resulted from the weathering of peridotite. 
The earthy masses are residual and the veins are deposits from down- 
ward percolating water that obtained nickel from the decomposing 
rock. 

The principal occurrences of garnierite are New Caledonia, where it is 
mined as a source of nickel, and at Riddles, Douglas Co., Oregon. A 
very closely allied species, genthite, occurs associated with chromite in 
serpentine at Texas, Lancaster Co., Penn., at Webster, N. C, at Malaga, 
in Spain, and at a few other places. 

Production. — Garnierite is mined from 40 mines on the plateau of 
Thio, New Caledonia, at the rate of about 130,000 tons annually of a 
6 J per cent ore. In 191 2 there were produced 72,315 tons of ore and 
5,097 tons of matte containing 2,263 tons 0I " nickel. The aggregate 
value of ore and matte was about $1,140,000. 



ANHYDROUS METASILICATES 401 

Meerschaum (H4Mg 2 Si 3 Oi ) 

Meerschaum, or sepiolite, occurs as a massive, dense, earthy aggre 
gate of a white, yellowish or reddish color, and also as a finely fibrous, 
crystalline aggregate (parasepiolile). It is opaque, has a conchoidal 
fracture and a shining white streak. Its hardness is 2 and density 
about 2. Dry specimens will float on water, because they are not 
easily wet. When touched to the tongue a clinging sensation is pro- 
duced. Two varieties of the commercial material have been recognized. 
Of these, one, a sepiolite, is HsMg 2 (Si30i 2 ) and the other p sepiolite, 
has the composition indicated above. 

The analyses of white meerschaum irom Asia Minor and from Utah 
gave the following results: 

Si0 2 AI2O3 Fe 2 3 MgO H 2 Total 

Asia Minor. . 52.45 .80 ... 23.25 23.50 100.00 

Utah 5 2 -97 -86 .70 22.50 18.70* 99-74 

* Of this 8.80% was driven off at ioo°. Included also are 3.14 Mn s Oj and .87 CuO. 

Before the blowpipe the mineral fuses on its edges to a white enamel. 
Often, at first, it turns brown or black v and then, upon higher heating, 
it bleaches to white. At low temperature in the closed tube it yields 
a little hygroscopic water. At high temperature water is given off freely. 
The mineral dissolves in hydrochloric acid, with the production of gelat- 
inous silica in the case of the a variety. 

Meerschaum resembles chalk and kaolin, from which it is easily dis- 
tinguished by treatment with hydrochloric acid. 

Occurrence and Localities. — The mineral is found as nodules in young 
sedimentary beds in Asia Minor, where it is associated with magnesite. 
Both minerals are believed to be alteration products of serpentine. It 
occurs also with opal at Thebes, Greece. A red variety occurs in lime- 
stone at Quincy, France, and a green and white variety forms a small 
vein in a silver ore in Utah. In all of its occurrences it seems to be 
secondary. 

Uses. — Meerschaum is used for carving into ornaments and pipes. 

Steatite (H 2 Mg 3 (Si0 3 )4) 

Steatite, or talc, usually occurs in flaky, foliated and massive forms, 
and in plates that appear to be tabular crystals with hexagonal outlines. 
It also forms, with chlorite and a few other substances, the rock soap- 
stone. Although its crystallization is unknown, because of the close 



402 DESCRIPTIVE MINERALOGY 

analogy between its physical properties and those of chlorite and the 
micas its symmetry is believed to be monociinic. 

The composition of pure white talc and ordinary soapstone are shown 
by the two analyses below: 

White talc Soapstone 

Uraerenthal, Switzerland W. Griqualand, Africa 

Si02 60 . 85 63 . 29 

AI2O3 1. 71 1.24 

Fe«203 .16 

FeO 09 4.68 

MgO 32.08 27.13 

H 2 495 4.40 

Total 99 . 68 100 . 90 

The composition corresponding to the formula H2Mg3(SiC>3)4 is: 
Si02=63.5, MgO = 3i.7 and H20=4.8. 

The cleavage of talc is well marked and on its cleavage surfaces its 
luster is pearly. Its cleavage plates are flexible. The mineral is white, 
gray, greenish or bluish, and is transparent or translucent. The massive 
forms, known as soapstone, are white, greenish, yellowish, red or brown. 
All varieties are soft — the mineral being chosen to represent 1 in the 
scale of hardness — and all have a soapy feeling. The density of pure 
talc is 2.6-2.8. For yellow light, a= 1.539, /3= 1.589, 7= 1.589. 

Before the blowpipe the mineral exfoliates, hardens and glows 
brightly, but it is nearly infusible (fusing temperature is about 1530 ), 
melting only on the thinnest edges to a white enamel. It yields water in 
the closed tube only at a high temperature. It is unattacked by acids 
before and after heating. Its powder reacts alkaline. 

It is distinguished from other white, soft minerals by its softness, its 
insolubility in acids and its infusibility. 

Occurrence. — The mineral is a common alteration product of other 
magnesium silicates, often pseudomorphing them. Thus, pseudo- 
morphs of the mineral after actinolite, bronzite and sahlite are common. 
Pseudomorphs after pectolite, dolomite and quartz are also known. In 
these forms it is secondary. 

It occurs also in marbles and other crystalline rocks, where it was 
produced by regional metamorphism. It is found, further, as small veins 
cutting serpentine and metamorphosed limestones, as layers under the 
name of talc schists, associated with other schistose rocks and as massive 
aggregates of finely matted fibers, probably resulting from the alteration 
of basic igneous rocks. The last described variety is the rock soapstone. 



ANHYDROUS METASILICATES 403 

The vein material is usually white, fibrous and pure. It is gi < und and 
placed on the market as talc. The impure variety (soapstone) is sawn 
into blocks and boards. 

Localities. — Talc and soapstone occur at many places. Good white 
platy talc occurs at Lampersdorf, in Silesia; near Pressnitz, in Bohemia; 
near Mautern, in Steiermark; at Andermatt, in Switzerland; at Russell, 
Gouverneur and other points in New York; at Webster, N. C; and at 
Easton, Penn. 

Uses. — Ground talc is extensively used as a lubricator, in the manu- 
facture of paper, as a filler in curtains, cloth, etc., as a foundry facing, in 
the manufacture of molded rubber goods, as a toilet powder, as a polish- 
ing material, as a pigment, in the manufacture of gas tips, pencils, cray- 
ons, etc. Soapstone is sawn and used as linings of acid vats and laundry 
tubs, and in the manufacture of table tops, sinks, etc., in chemical labora- 
tories. Because of its nonabsorbent qualities it is also being used 
largely in electric switchboards. Its various uses are due to its softness, 
infusibility, and its power of resistance to the attacks of acids. 

Production. — The principal sources of talc and soapstone in the 
United States are in a belt on the east side of the Appalachians ex- 
tending from Vermont to Georgia. Largest producers in 1912 were: 

Virginia, with a production of 25,313 tons, valued at $576,473, 
New York, with a production of 66,867 tons > valued at $656,270, 
Vermont, with a production of 42,413 tons, valued at $275,679. 

Of the aggregate of 159,270 tons, valued at $1,706,963 produced in 191 2, 
15,510 tons were sold* in the rough for $66,798; 2,642 tons, sawed into 
slabs, were sold for $50,334, 21,557 tons were manufactured and sold for 
$600,105, and 119,561 tons were sold ground for $989,726. Of this 
aggregate J 33> 2 ^9 tons > valued at $1,097,483 were talc and 25,981 tons, 
valued at $609,480 were soapstone. In addition to the home produc- 
tion, there were also consumed in the United States 10,989 tons of high- 
grade talc, valued at $122,956, which was imported. 

KAOLINITE GROUP 

The kaolinite group of minerals comprises hydrous aluminium sili- 
cates corresponding to the magnesium silicates of the serpentine group. 
The principal members of the group are: 

Kaolinite, H 4 Al 2 Si209, or H 2 Al(Al(OH)2)3(Si0 3 )4 

= 46.50 SK>2, 39.56 AI2O3, 13.94 H2O 
Pyropkyllite, H 2 Al 2 (Si0 3 )4 =66.65 Si0 2 , 28.35 AI2O3, 5.00 H 2 



404 DESCRIPTIVE MINERALOGY 

Kaolinite corresponds to serpentine in which all the Mg has been re- 
placed by Al and pyrophyllite to steatite. In addition to these, there 
are other closely related compounds which may be intermediate in com- 
position between these two. Among them the most common are alio- 
phane, monlmorillonite and haUoysite. 

Both minerals are of economic importance. Kaolinite is the base 
of all clay products like pottery, tile, bricks, etc. 

Kaolinite (H4Al 2 Si 2 9 ) 

The crystallization of kaolinite is probably monoclinic. The crystals, 
which are rare, are thin plates with an hexagonal habit, bounded by the 
planes oP(ooi), oo P(no) and oo P «> (oio) and +P(Tn). Their axial 
ratio is .5748 : 1 : 1.5997 with P=&$° n\ Their cleavage is perfect 
parallel to the base. 

Distinct crystals have been found only on the Island of Anglesey, 
Wales, and at the National Belle Mine, at Silverton, Colo., where they 
comprise a white powder every grain of which is a crystal. 

The mineral, when pure, is white or colorless and transparent. It 
has a hardness of 1 and a specific gravity of 2.45. It is infusible before 
the blowpipe and is only slightly attacked by HC1. It is decomposed 
by alkalies and alkaline carbonates with the production of hydrated 
silicates. Its index of refraction is about 1.56. 

The greater part of the kaolinite known is not in crystals. It usually 
occurs in foliated or dense earthy masses to which various names have 
been assigned. 

Nakrite is a white crystalline mass of kaolinite made up of tiny 
flakes often arranged in fan-like or divergent groups. The individual 
flakes have a pearly luster. It occurs as vein fillings in certain ore- 
bodies. 

Steinmarkite is a dense mass of microscopic grains often forming 
nodular masses and occurring as veins and nests in rocks. It is harder 
than pure kaolin (H= 2-3), and is often yellowish, gray or red in color. 

Kaolin is an earthy, friable mass of flaky kaolinite which when moist 
becomes plastic, and, therefore, of great value in the manufacture of 
pottery. It is more soluble in acids than the crystallized variety. It 
fuses at about 1780 . 

Kaolin is distinguished from chalk by its reaction toward HC1, from 
meerschaum and talc by the reaction for Al with Co(NC>3)2, and from 
infusional earth by the fact that its powder will not scratch glass. 

Clay is a mixture of kaolinite, quartz, fragments of other mineral 



ANHYDROUS METASILICATES 405 

particles and various decomposition products of kaolinite and other 
silicates, among the most important being various colloidal, hydrous, 
aluminous silicates and magnesium and calcium carbonates. The 
greater the proportion of colloidal material in the clay the more plastic 
it is and the more valuable for manufacturing purposes. Different clays 
have received different names which indicate in a way their uses. Among 
the most important of these are: 

China clay, a very pure, white kaolin, 
Ball clay, a white, very plastic clay, 
Fire clay, a fairly pure clay capable of resisting great heat, 
Flint clay, a hard clay which is not plastic even after grinding, 
Brick clay, an impure clay suitable for making brick, 
Pottery day, stoneware clay, terra-coUa clay, etc., are ?U impure clays 
that are adapted to the uses suggested by their names. 

Sample analyses of kaolinite and of some of the purer clays follow: 



Si0 2 


AI2O3 


Fe 2 03 


CaO 


Na 2 


H 2 


F 


Total 


I- 46.3S 


39-59 


.11 


•15 


• • • 


13-93 


■15 


100.13 


II. 46.86 


39 24 


• a • • 


• • • • 


• • • 


1390 


• • ■ 


100.00 


III. 43.46 


41.48 


• • • • 


1.20 


•37 


13-49 


• • • 


100.00 


IV. 59.92 


27.56 


io 3 


tr. 


64* 


10.82 


• • • 


99 97 



I. Crystals from National Belle Mine, Colo. 

II. Kaolin, Seilitz, near Meissen, Saxony. 

III. Steinmarkite, Schlaggenwald, Bohemia. 

IV. Flint fire clay, Salineville, Ohio. 

* NajO+KjO. 

Occurrence. — Kaolinite occurs in feldspathic rocks near ore veins. 
Here it was formed partly by ascending magmatic solutions and partly 
by descending H2SO4, produced by the oxidation of the sulphides in 
the upper portions of the veins. Most kaolin, however, is a weathering 
product of fefdspar (see p. 408), and of feldspathic rocks. When 
acted upon by water, and more particularly by water containing dis- 
solved CO2, the feldspars lose alkalies, calcium and some silica, leaving 
an aluminium silicate behind. Thus, for the potash feldspar orthoclase: 

K 2 • AI2O3 • 6Si0 2 ( = KAlSi 3 8 ) - K 2 • 4Si0 2 = AI2O3 • 2Si0 2 , which with 
2H2O = HiAl2Si209 (kaolinite) . 

Other silicates also yield kaolinite on weathering — in some cases 
completely changing so as to yield pseudomorphs of kaolin. 

Very complete weathering of this kind takes place in bogs, and 



406 DESCRIPTIVE MINERALOGY 

some of the best known beds of kaolin are believed to have been formed 
at the bottoms of peat bogs. 

Localities. — Kaolinite in measurable crystals occurs only at the two 
localities that have already been mentioned. The pure, white, dense 
kaolin is fairly widely spread. Clay occurs almost universally. The 
principal localities of kaolin in North America are near Jacksonville, 
Ala.; Mt. Savage, Md.; various points in Tennessee, North Carolina, 
Illinois, Missouri, New Jersey and Pennsylvania. 

Production. — The total value of clay products manufactured in the 
United States during 191 2 was over $172,800,000, of which by far the 
largest part is represented by common brick, of which $51,796,000 worth 
were made. Pottery followed with an output valued at $36,504,000. It 
is not possible to estimate the value of the clay represented in the man- 
ufactured product because in most cases the manufacturers mine their 
own clay and make no account of the raw material. The quantity of 
clay mined in the United States and sold to manufacturers during 191 2 
amounted to 2,530,000 tons, valued at $3,946,000. In addition, there 
were imported 334,655 tons of clay, valued at $1,952,000. 

Pyrophyllite (H 2 Al2(Si0 3 )4) 

> 

Pyrophyllite nearly always occurs in groups of radiating or diverging 
fibers that are either orthorhombic or monoclinic in crystallization. It 
may be isomorphous with steatite. The bundles of fibers cleave easily 
into flexible sheets that have a pearly luster on their cleavage faces. 
When pure the mineral is light-colored in shades of yellow, gray or green. 
It is transparent or translucent and has a greasy feel. Dense, struc- 
tureless masses are known as agalmatolite. 

The mineral is very soft, about 1. Its density is 2.8 or 2.9. Before 
the blowpipe it melts on the edges to a white enamel and fibrous varieties 
exfoliate and swell. Heated in the closed tube pyrophyllite assumes a 
silvery luster and gives off water. It is only partially soluble in HC1, but 
is completely decomposed by Na2CC>3. 

It is best distinguished from ta'c by the reaction for aluminium. 

Synthesis. — Upon heating to 3oo°-5oo° a mixture of Si02,Al203 and 
potassium silicate a mass is obtained which consists of andalusite, 
muscovite and pyrophyllite. 

Occurrence and Localities. — Pyrophyllite is found at a number of 
points in many different associations, where it is probably the result of 
weathering of other silicates. Its principal localities in the United 
States are Graves Mt., Ga.; Cotton Stone Mt., Deep River, Car- 



ANHYDROUS METASILICATES 407 

ft ' 

bonton and Glendon, N. C; Chesterfield, S. C; and Mahanoy City, 
Penn. 

Uses. — The massive form of the mineral is used to some extent in 
making slate pencils, and for the other purpose for which talc is employed. 
Agalmatolite is used by the Chinese as a medium from which they carve 
small images. 



CHAPTER XVIII 
THE SILICATES— C<wtf»M«* 

THE ANHYDROUS TRIMETASILICATES 

THE FELDSPARS 

The feldspars are among the most important ot all minerals. They 
are abundant as constituents of many igneous rocks and in mixtures 
filling veins. Their principal scientific importance lies in the fact that 
they indicate by their composition the nature of the rock magmas from 
which they crystallize. Consequently, in some systems of rock classi- 
fication the grouping of the rocks is based primarily upon the presence 
or absence of feldspar, and the naming of the feldspathic rocks is in 
accordance with the nature of their most prominent feldspathic con- 
stituent. Moreover, some of the feldspars are of economic importance. 

Chemically, the feldspars may be regarded as isomorphous mixtures 
of the four compounds, KAlSiaOg, NaAlSiaOg, Na2AlAlSi20g, CaAl AlSi20s 
and BaAlAlSi^s, each of which, except the third, has been found nearly 
pure in nature as orthoclase and microcline y barbieriU and albite, an- 
orthite and celsian. The third, Na2AlAlSi20g, has been made in the 
laboratory, but it occurs in nature only in isomorphous mixtures with 
the anorthite and albite molecules. The pure compound has been 
called carnegieite and its mixtures anemousites. The feldspars have 
also been regarded as salts of the acid HsAlSi208 in which the hy- 
drogen is replaced by various radicals, thus: (KSi)AlSi20s, orthoclase; 
(NaSi)AlSi 2 8 , albite; (CaAl)AlSi 2 8 , anorthite, and (BaAl)AlSi 2 8 , 
celsian. 

The potash molecule crystallizes from magmas containing potas- 
sium, sodium and calcium, but it also frequently forms isomorphous 
mixtures with the soda molecule and in some cases with the barium 
molecule. Mixtures of the potash and calcium molecules are ex- 
tremely rare as minerals, but they have been formed experimentally 
in the laboratory. The albite and the calcium molecules are usually 
intermixed. Both are known in a nearly pure condition as minerals, 
but their mixtures are much more common. Indeed they are so common 
that they are separated from the other feldspars and formed into a dis- 

408 



ANHYDROUS TRIMETASIUCATES 409 

tinct subgroup under the name of the plagioclase group, with albite and 
anorthite as the two end members. The plagioclases constitute the best 
known isomorphous series of compounds in the realm of mineralogy. 

The calculated compositions of pure orthoclase (or microcline), 
albite, anorthite and celsian with their specific gravities are: 

Na 2 CaO BaO Sp. Gr 

• • • • •••• •••• * . s s 

II .8 .... .... 2 .61 

.... 20 . 1 .... 2 . 76 

.... .... 41 .0 3 • 34 





Si0 2 


AI2O3 


K 2 


Orthoclase. 


■ 64.7 


18.4 


16.9 


Albite 


68.7 


19 5 


• • • • 


Anorthite. . . 


43-2 


36.7 


• • • • 


Celsian 


32.0 


27.2 


• • • • 



All the feldspars are triclinic, but the pure potassium and sodium com- 
pounds, in addition to possessing distinct triclinic phases (microcline and 
albite) occur also in crystals which, because of sub-microscopic twinning, 
(p. 420) are apparently monoclinic (orthoclase and barbierite). Usually 
the forms on orthoclase are designated by symbols that refer to the 
monoclinic axes, but since the habits of all feldspars are the same they 
can be as readily understood when referred to the triclinic axes. The 
crystallographic constants for the members of the group that consist 
of unmixed molecules are: 



Orthoclase. . 1 ^ , / o 

Microcline K^s : i : -5554 90° «6° 3' 90° 



a b c a y Angle(ooi)A(oio) 

90° 
89 30' 

Celsian 657 : 1 : .554 90 115 2' 90 90 

Albite 6335 : 1 : .5577 94° 3' "6° 29' 88° 9' 86° 24' 

Anorthite 6347 : 1 : .5501 93 13' 115 53' 91 12' 85 50' 



lase. . 1 
line. . J ' 



The simple crystals of feldspar exhibit three habits, but on nearly all 
the same forms occur. These are oP(ooi), 00 P 60 (010), ooP'(no), 
00 ; P(iTo), /P/60 (Toi), 2/P/ 60 (201) and less commonly 2/P' 00(021), 
2 ; P, 66 (021), 00 P/3(i3o), 00 /P3(i3o), ,P(Tn), P,(T7i) and 00 P 60 (100). 
In orthoclase and the other apparently monoclinic forms these symbols 
may be written oP, 00 P 00 , 00 P, Poo, 2P00, 2P00, 00 P3, P and 
00 P66 (Figs. 213 and 214). There have, moreover, been reported on 
orthoclase about 90 other planes and on the plagioclases about 45. Of 
these, however, a number are probably vicinal, as they have extremely 
large indices. 

The principal habits are the equidimensional, the columnar (Fig. 
213), and the tabular (Fig. 214). The tabular crystals are usually 
flattened parallel to 010. The columnar forms are elongated parallel 
to the c or the a axes. 



410 



DESCRIPTIVE MINERALOGY 



Twinning is common, according to five laws, and much less common 
according to several others. Of the five common laws three apply to all 
the feldspars, and the remaining two to the triclinic types alone. The 
first three are the Carlsbad, the Manebach and the Baveno. The other 
two are the albite and the pericline. 

In Carlsbad twins, ioo is the twinning plane and usually oio is the 




TO 



m 



V 



V 







Fig. 214. 



Fig. 213. 

Fig. 213. — Orthoclase Crystals with 00 P, no (m); *> P *> . 010 (6); oP, 001 (c) and 

2P « , 201 (y). 

Fig. 214. — Orthoclase Crystals with m, b, c and y as in Fig. 213. Also P « , Toi (x); 

P, In (a); °o P$» 130 (2) and 2P& , 021 (n). 




m 



m 





Fig. 215. 



m\ 



m 



«l 



ksA/ 



Fig. 216. 



Fig. 215. Carlsbad Interpenetration Twins of Orthoclase. Twinning plane is <» P 00 

(100); composition face ooPoj (010). 

Fig. 216. — Contact Twin of Orthoclase According to the Carlsbad Law. 



composition face. The twinned parts may interpenetrate, as is usually 
the case (Fig. 215), or they may lie side by side forming a contact twin 
(Fig. 216). If in the contact twins the planes Toi and 001 are equally 
prominent, since they are nearly equally inclined to the c axis the twin 
may be mistaken for a simple crystal (Fig. 216). In rare cases the 
composition face is 100 and the twinned parts are in contact. 



ANHYDROUS TRIMETASILICATES 411 

The Baveno twins are contact twins, with 021 the twinning and com- 
position planes (Fig. 217). As the individuals are elongated parallel to 
the a axis the result of the twinning is a square prism with its ends 
crossed by a diagonal that separates the same forms on the two twinned 
individuals. In some cases the twinning is repeated and a fourling 
results. 

In Manebach twins, the twinning and composition plane is 001. 
These usually occur in columnar crystals elongated parallel to a, or in 
tabular crystals flattened parallel to 001 or 010 (Fig. 218). 

Carlsbad, Baveno and Manebach twins, as has been stated, are com- 
mon to feldspars of both the monoclinic and triclinic phases, but the 
pericline and albite laws are found only in the triclinic types. The 





Fig. 217. Fig. 218. 

Fig. 217. — Baveno Twin of Orthoclase. Twinning and composition plane, 2P 5b (021). 
Fig. 2 1 8. — Manebach Twin of Orthoclase. Twinning and composition plane, oP(ooi). 

description of these is, therefore, deferred until the plagioclases are dis- 
cussed. 

Besides occurring in crystals, nearly all the feldspars are known also 
in granular and platy masses. 

The pure feldspars are colorless and transparent or translucent, and 
all have a glassy luster which, on cleavage faces sometimes approaches 
pearly. As usually found, the feldspars are white, pink, reddish, yellow- 
ish, gray, bluish or green. Some specimens show a bluish white shimmer 
or opalescence (moonstone), and others a reddish sparkle (sunstone), 
due to enclosures of other minerals or of lamellae of a different refractive 
index from that of the main portion of the mass. All have a white 
streak. All possess a very perfect cleavage parallel to the base (001) 
and a scarcely less perfect one parallel to 010. Their fracture is uneven 
to conchoidal, and hardness 6. 

Before the blowpipe fragments of the potash, barium, and calcium 



412 DESCRIPTIVE MINERALOGY 

feldspars are very difficultly fusible on their ed^es to a porous glass. 
The soda feldspars are a little more easily fusible. The fusing tempera- 
ture of albite is between 1200 and 1250 , that of orthoclase approxi- 
mately 1300 , and that of anorthite 1532 . Anorthite is soluble in 
hydrochloric acid with the production of gelatinous silica. The other 
three feldspars are insoluble. 

The feldspars are distinguished from other minerals by their crys- 
tallization, their two nearly perfect cleavages approximately perpen- 
dicular to one another, and their hardness. They are distinguished 
from one another by characters that will be indicated in the descriptions 
of the several varieties. 

Feldspars rich in orthoclase and soda weather fairly readily to mus- 
covite, or kaolin and quartz. The soda feldspars in some cases change 
to zeolites (p. 445). With the addition of the calcium molecule calcite 
is often found in the weathering products. Under certain conditions, 
especially when in rocks containing magnesium and iron minerals, the 
calcium feldspars often change to a mixture of zoisite and albite, or a 
mixture of these with garnet, chlorite (p. 428), epidote and other com- 
pounds. This mixture is often designated by the name saussuriie. 

Syntheses. — All crystals of the feldspars, except those of pure albite 
and pure orthoclase (including microcline), have been made by slowly 
cooling a dry fusion of their components in open crucibles. Albite and 
orthoclase have been produced from similar fusions to which tungstic 
acid, alkali-tungstates or phosphates, or alkali-fluoride have been added. 
They have also been produced with quartz by fusion in the presence 
of moisture in closed tubes. 

Occurrence and origin. — All except the barium feldspars occur as 
important constituents of most igneous and of many metamorphic 
rocks. They occur also abundantly ifi a few sandstones (arkoses) and 
in a few water-deposited veins, and are found around a few volcanic 
craters as products of gaseous exhalations. The barium feldspars are 
rare. They have been seen only in dolomite associated with barite and 
tourmaline, in manganese ores and manganese epidote; and intergrown 
with albite in a pegmatite at Blue Hill, Delaware Co., Pa. 

With respect to origin feldspars may be primary separations from a 
magma, primary deposits from solutions, pneumatolytic deposits, or 
they may be the result of metasomatic process. They are common 
products of contact and regional metamorphism. 

Uses. — The feldspars, though extremely abundant, have compara- 
tively few uses. In the future the potash varieties may become a 
source of the potash salts used in the manufacture of fertilizers. At 



ANHYDROUS Till MET ASILICATES 413 

present the principal use of the feldspars is in the manufacture of por- 
celain and other white pottery products and enamel ware. They are 
used as fluxes to bind together the grains of emery and carborundum 
in the making of grinding and cutting wheels, and are employed also in 
the manufacture of opalescent glass, artificial teeth, scouring soaps and 
" ready roofing." 

Production. — All the feldspar used in commerce comes from pegma- 
tites. The total quantity produced for all purposes in the United States 
during 191 2 amounted to 86,572 tons, valued at $520,562. Of this, 
26,462 tons were sold crude at a value of $89,001 and the balance 
ground. The principal varieties mined are orthoclase, microcline and 
albite, though oligoclase (a plagioclase rich in soda) is mined in small 
quantity. 

ALKALI FELDSPARS 

Orthoclase and Microcline (KAlSi 3 8 ) 
Barbierite and Albite (NaAlSisOg) 

Orthoclase and microcline have the same chemical composition. 
Both are potash feldspars, but both may contain sodium. On the 
other hand barbierite and albite are both essentially soda feldspars but 
both usually contain some potassium. In orthoclase the sodium is 
due to the admixture of the barbierite molecule, and in microcline to 
the presence of the albite molecule. The soda-rich microcline is gen- 
erally known as anorthoclase. The pure barbierite is not known to 
exist as a mineral. Analyses of these four varieties follow: 



Si0 2 


A1203 


CaO 


K 2 


Na 2 


H 2 


Total 


I. 63.80 


21.00 


a • • a 


13.80 




1.40 


100.00 


II. 65.23 


19-35 


.76 


931 


4-52 


.27 


100.00 


III. 67.00 


19. 12 


.78 


115 


11.74 


• • • * 


99-79 


IV. 66.18 


i9S 2 


■36 


13-03 


.91 


• • • • 


100.00 


V. 67.99 


19.27 


•75 


3 05 


6.23 


.00 


99 03 


VI. 68.28 


19.62 


•3i 


•39 


10.81 


.09 


99.82 



I. Orthoclase, Adularia, Elba. 
II. Soda-orthoclase, Drachenfels, Prussia. Also .56 BaO. 

III. Barbierite, Krager8, Norway. 

IV. Microcline, Ersby, Pargas, Finland. 

V. Anorthoclase, from granite, Kekequabic Lake, Minn. Also .82 Fe»Oi and 

trace of MgO. 
VI. Albite, from litchfieldite. Litchfield. Maine. Also .23 FeO and .09 MgO. 

Albite is described among the plagioclases (p. 418). 



DESCRIPTIVE MINERALOGY 



The most noticeable difference between orthoclase and mkrocline 
is that the latter shows dearly its triclinic symmetry by its twinning, 



Fig. 119.— Section of Microcline Viewed between Crossed Nicols. The grating 
structure indicates twinning. (Aflrr Rosrnbiisch.) 

and its optical properties, while in orthoclase the twinning is so 
minute as to be unobservable and the optical properties are similar to 
those of monoclinic crystals. This difference is best exhibited in thin 
sections when viewed ia polarized light under the 
microscope. Under these conditions certain sec- 
tions of microcline exhibit series of light and dark 
bars crossing one another perpendicularly (Fig. 
219), while sections of orthoclase do not. The 
grating structure is due to repeated twinning 
according to the albite and pericline laws at the 
same time (p. 419). If this method of twinning 
is present in orthoclase the lamellae are so 
minute that they cannot be seen even under 
high powers of the microscope. 

Several names that refer to more or less dis- 
tinct varieties of the potash feldspars are in com- 
mon use. The most important are: 
Adularia, a nearly pure orthoclase, that is nearly transparent, occur- 
ring in veins. Its crvstals have the characteristic habit illustrated in 
Fig. no. 



Fig. ?70. — Adularia 
Crystal with «, b. 



Figs. 213 and 214. 
Also !P 3B . I03 (q). 



ANHYDROUS TRIMETASILICATES 415 

• 

Sanidine, a glassy soda orthoclase, occurring as large crystals often 
flattened parallel to oio, embedded in lavas. 

Moonstone, a translucent adularia, exhibiting a pearly luster, with 
a very slight play of colors. 

Sunstone, a translucent variety exhibiting reddish flashes from 
inclusions of mica, or other platy minerals. 

Perthite, parallel intergrowths of thin lamellae of orthoclase and 
albite. 

Microcline- perthite, parallel intergrowths of lamellae of microcline 
and albite. 

Orthoclase and the other pseudomonoclinic feldspars may be dis- 
tinguished from the distinctly triclinic forms by the value of the cleavage 
angle which in orthoclase is 90 , and in the triclinic forms about 86°, 
except in microcline. (See p. 409.) The value of the angle 110A1T0 
= 6i° 13' in orthoclase. Its refractive indices for yellow light are: 
a=i.5ig, &= 1.524, 7 = 1.526. With the admixture of the albite mole- 
cule these values increase. The sp. gr. of pure orthoclase is 2.55 and 
its fusing point a little higher than that of albite (see p. 412). 

Orthoclase may be distinguished from the other pseudomonoclinic 
feldspars by its specific gravity and the flame reaction. 

Syntheses. — Crystals of orthoclase have been made by fusing 
S1O2 and AI2O3 with potassium wolframate, vanadate or phosphate. 
Also by heating aluminium silicate with a solution of potassium silicate 
and KOH in a tube at ioo°, and by heating muscovite in a solution of 
potassium silicate at 6oo°. 

Occurrence. — The potash feldspars are essential constituents of the 
igneous rocks — granite, syenites, rhyolites and trachytes — and of some 
crystalline schists, and are accessory components of a number of other 
rocks. They occur in most pegmatite dikes and as gangues in some ore 
veins, and in many contact metamorphosed rocks. 

Localities. — The potash feldspars are so widely spread that an enu- 
meration of their important occurrences is here impossible. The best 
known localities of orthoclase are Cunnersdorf, Silesia; Drachenfels 
and Lake Laach, Rhenish Prussia (sanidine) ; in the Zillerthal, Tyrol 
(adularia); at St. Gothard in the Alps (adularia); at Baveno, Italy, 
and at Mt. Antero, Chaffee Co., Col. Microcline crystals are well 
developed at Striegau, Silesia; in the pegmatite dikes of southern Nor- 
way; and at Pike's Peak, Col. (amazonite). Anorthoclase occurs at 
Tyveholmen and other points in Norway and in the lava of Kilimand- 
jaro, Africa, and in that on Pantelleria, an island near Sicily. In 
North America pegmatites are abundant in southeastern Canada, in 



416 DESCRIPTIVE MINERALOGY 

New England and in the Piedmont plateau area immediately east of 
the Appalachian Mts., and throughout this district all forms of the 
opaque potash feldspars are abundant. Soda-potash feldspars have 
been described from many places, but whether they are soda orthoclase 
or anortholcase has rarely been determined. 

All phases of the alkali feldspars occur as components of igneous 
and metamorphic rocks. 

POTASH-BARIUM FELDSPARS 

The feldspars containing potassium and barium comprise an iso- 
morphous series with orthoclase and celsian as the two end members as 
follows: 







Sp. Gr. 


Orthoclase (Or) 


KAlSiaOg 


2-55 


Barium orthoclase 


OrioCei — OrioCei 


2 . 593-2 . 645 


Hyalophane 


Or 4 Cei-Or 7 Ce 3 i 


2.725-2.818 


Celsian (Ce) 


BaAl 2 (Si0 4 ) 2 


3384 



The chemical composition of some of the barium feldspars are illus- 
trated by the analyses quoted below: 

Si0 2 AI2O3 BaO CaO MgO K 2 Na 2 H 2 Total 

I. 51.68 21.85 16.38 10.09 100.00 

II. 52.67 21.12 1505 .46 .04 7.82 2.14 .58 99.88 

III. 53.53 23.33 7.30 3.23 11. 71 ... 99.10 

IV. 54.15 29.60 1.26 1. 00 1.52 12.47 ... 100.00 

I. Theoretical for Or 2 Cei. 
II. Binnenthal, Tyrol. 

III. Jakobsberg, Sweden. 

IV. Sjogrufran, Sweden. 

The minerals are isomorphous with orthoclase (with the possible 
exception of celsian, which may exhibit the triclinic habit and may more 
properly be isomorphous with microclinic), and their axial constants are 
intermediate between those of orthoclase and celsian. The axial ratio 
for hyalophane is .6584 : 1 : .5512. a=9o°, /3=ii5° 35', 7 = 90°. Its 
cleavage angles are 90 . Its crystals, as a rule, have the adularia habit. 
The Indices of refraction of the barium feldspars are: 







a 





y 


Barium-orthoclase 


•(OrioCei) 


I.5201 


1.5240 


I-5257 


Hyalophane 


(OnCei) 


1-5373 


1-5395 


1. 5416 


Hyalophane 


(Or 7 Ce 3 ) 


i.54i9 


I-54I9 


1 5469 


Celsian 




1 ■ 5837 


1 . 5886 


1 • 5940 



ANHYDROUS TRIMETASILICATES 



417 



These feldspars are rare. They have been found only in metamor- 
phosed dolomites in the Binnenthal, Valais; at the manganese mines at 
Jakobsberg and Sjogrufran, Sweden; and intergrown with albite in a 
pegmatite at Blue Hill, Delaware Co., Penn. 

SODA-LIME FELDSPARS 

Plagioclase is the general name given to the group of isomorphous 
feldspars of which albite and anorthite are the end members. The 
albite and anorthite molecules are isomorphous in all proportions and 
the physical properties of the mixed crystals accord completely with 
their composition. Certain mixtures are much more common than 
others. These were given individual names before it was recognized 
that they were merely members of an isomorphous series and these 
names were later applied to mixtures of definite compositions. The 
names and the compositions of the mixtures corresponding to them are 
given in the following table. 



Si0 2 

Albite NaAISi 3 8 (Ab) 68. 7 



Oligoclase 



Andesine 



Labradorite 



Bytownite 



AbftAni 
AbsAni 

AbsAni 
AbiAni 

AbiAni 
AbiAri3 

AbiAn3 
AbiAntj 



64.9 
62.0 



AI2O3 

19-5 
22.1 

24.0 



55° 28 3 



49-3 32-6 



Na 2 

11. 8 
10. o 

87 

5-7 



CaO 

■ ■ • ■ 

3-0 

5-3 
10.4 



Sp. Gr. 
2.605 

2.649 
2.679 



2.8 15.3 2.708 



46.6 



Anorthite CaAkCSiO-iMAn) . . 43 . 2 



34 4 
36.7 



1.6 



17-4 
20.1 



2.742 
2.765 



Nearly all plagioclases contain small traces of K2O, MgO and Fe203, 
but otherwise their composition is nearly in accord with that demanded 
by their symbols, so that if one constituent is known the others may 
be calculated. Moreover, the accord between physical properties and 
composition is so close that from the former the latter may be de- 
termined. 

Many oligoclases, however, contain a large admixture of the micro- 
cline molecule so that they contain a notable quantity of KoO. These 
are known as potash-oligodase and are represented by the feldspar in a 
rock at Tyveholmen, Norway, the composition of which is as follows: 

Si0 2 AI2O3 Fe 2 3 CaO MgO K 2 Na 2 H 2 Total 
59.50 22.69 2.47 5.05 tr. 2.50 6.38 1.37 100.37 



418 DESCRIPTIVE MINERALOGY 

Some authors limit the name anorthoclase to feldspars of this kind and 
designate the triclinic soda-potash feldspar as soda-microcline. 

There is another group of soda-lime feldspars in which the anorthite 
molecule and an analogous sodic molecule (Na2Al2(Si04)2) form iso- 
morphous mixtures. The pure sodic molecule has not been found among 
minerals, but it has been prepared synthetically at temperatures above 
1248 , under the name carnegieite. Its sp. gr. = 2.513 and its refractive 
indices for yellow light are: a= 1.509, 7=1.514. Although not known 
to exist independently it is believed to be present in the feldspar of 
Linosa, near Tunis, and possibly in other feldspars that have hitherto 
been described as plagioclases. If future work establishes the fact that 
there is a distinct series of feldspars composed of isomorphous mixtures 
of anorthite and carnegieite it is proposed to name the group anemousiie 
to distinguish it from the plagioclase group which comprises isomorphous 
mixtures of anorthite and albite. 

The Linosa feldspar has properties nearly like those of the plagioclase 
AbiAni but its analysis yields the results in line I. The composition of 
AbiAni is given in line II. 



Si02 


AI2O3 


CaO 


Na 2 


K 2 


Sp. Gr. 


L 53.26 


29.78 


10.76 


5-45 


•75 


2.684 


IL 55 67 


28.26 


10.34 


5 73 


.00 


2.679 



THE PLAGIOCLASES 

All the plagioclases have a triclinic habit, which is best expressed by 
the value of the angle between their cleavages, which are parallel to 
the planes 001 and" 010. The crystal constants of some of the common 
mixtures and the values of their cleavage angles are given in the table 
below. 

Angle 

ol p y 

001A010 

.5577 94° 3' n6°2 9 ' 88° 9' 86° 24' 
•55 2 4 93° 4' ii6 23' 90 5' 86° 32' 
.5521 93 23' n6°2 9 ' 8 9 °59' 86° 14' 

•5547 93° 3i' n6°3' V55' 86° V 

= .6347 : 1 : .5501 93° 13' «S° 55' 91° "' ^5° 5°' 

Crystals of the soda-rich plagioclases are rich in forms, but those of 
anorthite and the lime-rich members are much simpler. Albite crystals 
are usually tabular parallel to 00 P 66 (010) and elongated parallel to 
c or a. Others are elongated parallel to b (Fig. 221). Oligoclase is 



Albite a : b : 


£=•6335 : 1 


Oligoclase. . . 


= .6321 : 1 


Andesine. . . 


= .6357 : 1 


Labradorite. 


= .6377 : 1 


Bytownite. . 




Anorthite. . . 


= .6347 : 1 



ANHYDROUS TRIMETASILICATES 



419 



more frequently columnar parallel to c, andesine tabular parallel to 
oo P 06 (oio) or oP(ooi), and labradorite and bytownite tabular parallel 
to oo P oo (oio). Twins are even more common than among the potash 
feldspars. Carlsbad (Fig. 
222), Manebach and Ba- 
veno twins are not uncom- 
mon, but more frequent 
than these are the twins 
after two laws that are 
impossible in the feld- 
spars with a monoclinic 
habit. The two most 
common twinning laws 
among the plagioclases are 
the albite and the pericline 
laws. 

In the albite law the twinning plane is oo P 6o (oio) and the com- 
position plane the same (Fig. 223). The twinning is usually repeated 
many times so that apparently homogeneous crystals may be built up 
of numerous lamellae paraUel to 010. Since the angle between 010 and 





Fig. 221. — Albite Crystals with 00 'P, 1T0 (Af); 
00 P', no (in); 00 P 00 , 010 (6); oP, 001 (c) and 
,P, 00 , Toi (x). 




Fig. 222. 




Fig. 222. — Albite Twinned about 00 Poo, 100. Composition face 00 Poo, 010. 

Carlsbad law. Compare Fig. 216. 

Fig. 223. — Albite Twinned about 00 Poo, 010. Composition face the same. Albite 

law. Compare Fig. 222. 



001 in all the plagioclases is greater and less than oo°, it must follow that 
the surface of their basal cleavages is not a plane, but that it consists of 
parallel strips of surfaces parallel to 010, and inclined to one another at 
angles alternately greater and less than 180 .. Therefore basal cleavages 



420 



DESCRIPTIVE MINERALOGY 



of the plagioclases very frequently exhibit parallel striations when exam- 
ined in light reflected at the 
proper angles (Fig. 224). 
It is this twinning which, 
repeated in sub microscopic 
lamellae, is believed to pro- 
duce the monoclinic pseudo- 
symmetry of orthoclase. 
It will be noted that the 
twinning plane has the 
position of the plane of 
Fig. 124.— Twinning Striations on Cleavage Piece symmetry in monoclinic 
of Oligoclase. (About natural size.) crystals, and, consequently, 

twins about this plane have 
the same symmetry with reference to one another as corresponding 
contiguous layers of mono- 
clinic crystals. 

In the periclinc law the 
twinned portions are super- 
posed. The individuals are 
twinned about b as the twin- Fig. 2!5.— Albite Twins with the Crystal Axis 
ningaxis,and are united about 6 the Twinning Axis and the Rhombic Sec- 
a plane nearly perpendicular tbn the Composition Face. The form r is 

r. - / \t .u 4.P« (403). Perkline law. 

to 00 P 00 (010), known as the ' ™ J 

" rhombic section " (Fig. 225). The position of this section varies 
with the different plagioclases, but is always nearly perpendicular to 010 






Fig. 226. Fig. 127- 

Fig. 226.— Position of " Rhombic Sections " in Albitc (.1) and Anorthite (B). 
Fie. 227.— Diagram of Crystal of Tridinic Feldspar Exhibiting Striations Due to 
Polysynthetic Twinning According to the Albitc and the Peridine Laws. 

(Fig. 226). As nearly all pericline twins are elongated in the direction 
of the b axis, and the twinning is repeated, lamellae are produced, 



ANHYDROUS TRIMETASILICATES 421 

which, in sections perpendicular to oio, cross the albite lamellae at 
angles near 90 (Fig. 227). It is the presence of the two kinds of 
twinning in microcline that gives it its peculiar grating structure in 
polarized light (see Fig. 219). 

The plagioclases are light-colored, but pinkish and greenish shades 
are less common in them than in the potash feldspars. Their streak is 
colorless. They are usually translucent but in some cases are trans- 
parent. Albite often exhibits a pearly luster and often a bluish shimmer. 
Oligoclase when containing as little inclusions plates of hematite, glistens 
with a red shimmer and affords the finest sunstones. The most bril- 

* 

liantly colored plagioclases are some forms of labradorite, which, on 
cleavage surfaces, show a great display of yellow, green, red, purple and 
blue flashes in reflected light. The cause of the play of colors is not 
known, but it is probably due to the presence of numerous very tiny 
paral